Propolis is a resinous substance that bees collect from tree buds or other botanical sources. It is used as a sealant for unwanted open spaces in the hive. Propolis is used for small gaps (approximately 6.35 millimeters (0.3 in) or less), while larger spaces are usually filled with beeswax. Its color varies from green to reddish brown depending of its botanical source; the most common being dark brown.
For centuries, beekeepers assumed that bees sealed the beehive with propolis to protect the colony from the elements, such as rain and cold winter drafts. However, 20th century research has revealed that bees not only survive, but also thrive, with increased ventilation during the winter months throughout most temperate regions of the world.
Propolis is now believed to:
reinforce the structural stability of the hive
make the hive more defensible by sealing alternate entrances
prevent diseases and parasites from entering the hive
prevent putrefaction within the hive. Bees usually carry waste out of and away from the hive. However if a small lizard or mouse, for example, found its way into the hive and died there, bees could be unable to carry it out through the hive entrance. In that case, they would attempt instead to seal the carcass in propolis, essentially mummifying it and making it odorless and harmless.
Propolis is marketed by health food stores as a traditional medicine, and for its claimed beneficial effect on human health. Natural medicine practitioners often utilize propolis for the relief of various conditions, including inflammations, viral diseases, ulcers, superficial burns or scalds. Some such therapies are based on the traditional practices of Traditional Chinese Medicine, ayurveda or homeopathy. Propolis is also believed to promote heart health and reduce the chances of cataracts.Old beekeepers recommend a piece of propolis kept in the mouth as a remedy for a sore throat. Propolis lozenges can be bought in France. Though claims have been made for its use in treating allergies, propolis may cause severe allergic reactions if the user is sensitive to bees or bee products. .
Few of these folkloric claims have been clinically evaluated at the level of large-scale, randomized, double-blind studies. Some in-vitro or rat-model studies are available in published biomedical literature. The composition of propolis is variable, depending on season, bee species and geographic location, so caution must be applied in extrapolating results(below). Depending upon its precise composition propolis may show powerful local antibiotic and antifungal properties.  Studies indicate that it may be effective in treating skin burns.  Propolis also exhibits immunomodulatory effects.
Propolis has attracted the attention of the dental community, although there is currently no conclusive evidence to support the argument that propolis actively protects against caries and other forms of oral disease. Propolis can also be used to treat canker sores , and its use in canal debridement for endodontic procedures has been explored in Brazil.
Propolis is used by certain music instrument makers to enhance the appearance of the wood grain. It is a component of Italian varnish and was reportedly used  by Stradivari
The composition of propolis will vary from hive to hive, district to district, and from season to season. Normally it is dark brown in color, but it can be found in green, red, black and white hues, depending on the sources of resin found in the particular hive area. Bees are opportunists, and will gather what they need from available sources. Occasionally bees will even gather various caulking compounds of human manufacture, when the usual sources are more difficult to obtain. Therefore, various potential medicinal properties may be present in one hive's propolis and absent from another. The properties of the propolis depend on the exact plant sources used by an individual hive, and the distributors of propolis products cannot control such factors. This may account for the many and varied claims regarding its potential medicinal properties and the difficulty in replicating previous scientific studies investigating these claims). Even propolis samples taken from within a single colony can vary, making controlled clinical tests virtually impossible.
The source of propolis varies in a major way with latitude. In temperate climates bees collect resins from trees, mostly poplars and to a lesser extent conifers. The biological role of propolis in trees is to seal wounds and defend against bacteria, fungi and insects. In tropical regions, bees gather propolis from flowers, especially Clusia, that have adapted propolis to attract pollinators. The chemical composition of temperate propolis and tropical propolis are different. Poplar propolis is rich in flavanoids. Clusia propolis contains polyprenylated benzophenones.  In Chile propolis contains viscidone, a terpene from Baccharis species . "Typical" propolis has approximately 50 constituents, primarily resins and vegetable balsams (50%), waxes (30%), essential oils (10%), and pollen (5%). Propolis is sticky at and above room temperature. At lower temperatures it becomes hard and very brittle.
"Sinapic acid, isoferulic acid, caffeic acid and chrysin were isolated from the alcoholic extraction of propolis and identified by spectrometric methods. The first three compounds were shown with inhibitive effect of against Staphylococcus aureus, while chrysin was ineffective." 
 R Krell 1996. value-added products from beekeeping FAO AGRICULTURAL SERVICES BULLETIN No. 124 Food and Agriculture Organization of the United Nations Rome
 Effects of some probable antioxidants on selenite-induced cataract formation and oxidative stress-related parameters in rats. Toxicology1999 Dec 6;139(3):219-32
 2 cases of allergic reaction after administration of propolis drugs
 Susceptibility profile of Salmonella against the antibacterial activity of propolis produced in two regions of Brazil 
 http://www.ncbi.nlm.nih.gov/sites/entrez?Db=pubmed&Cmd=ShowDetailView&TermToSearch=11890438&ordinalpos=3&itool=EntrezSystem2.PEntrez.Pubmed.Pubmed_ResultsPanel.Pubmed_RVDocSum Abstract Comparison of propolis skin cream to silver sulfadiazine: a naturopathic alternative to antibiotics in treatment of minor burns.
 http://www.ncbi.nlm.nih.gov/sites/entrez?Db=pubmed&Cmd=ShowDetailView&TermToSearch=15019118&ordinalpos=2&itool=EntrezSystem2.PEntrez.Pubmed.Pubmed_ResultsPanel.Pubmed_RVDocSum The effect of CAPE on lipid peroxidation and nitric oxide levels in the plasma of rats following thermal injury
 http://www.ncbi.nlm.nih.gov/sites/entrez?Db=pubmed&Cmd=ShowDetailView&TermToSearch=16828884&ordinalpos=1&itool=EntrezSystem2.PEntrez.Pubmed.Pubmed_ResultsPanel.Pubmed_RVDocSum Role of caffeic acid phenethyl ester, an active component of propolis, against NAOH-induced esophageal burns in rats.
 http://www.ncbi.nlm.nih.gov/sites/entrez?Db=pubmed&Cmd=ShowDetailView&TermToSearch=10575279&ordinalpos=18&itool=EntrezSystem2.PEntrez.Pubmed.Pubmed_ResultsPanel.Pubmed_RVDocSum [Prophylactic effectiveness of propolis for immunostimulation: a clinical pilot study
 http://www.ncbi.nlm.nih.gov/sites/entrez?Db=pubmed&Cmd=ShowDetailView&TermToSearch=12939048&ordinalpos=16&itool=EntrezSystem2.PEntrez.Pubmed.Pubmed_ResultsPanel.Pubmed_RVDocSum Propolis and some of its constituents down-regulate DNA synthesis and inflammatory cytokine production but induce TGF-beta1 production of human immune cells.
 http://www.ncbi.nlm.nih.gov/sites/entrez?Db=pubmed&Cmd=ShowDetailView&TermToSearch=15354462&ordinalpos=9&itool=EntrezSystem2.PEntrez.Pubmed.Pubmed_ResultsPanel.Pubmed_RVDocSum A clinical study of a silicate toothpaste with extract from propolis.
 http://www.ncbi.nlm.nih.gov/sites/entrez?Db=pubmed&Cmd=ShowDetailView&TermToSearch=12459618&ordinalpos=14&itool=EntrezSystem2.PEntrez.Pubmed.Pubmed_ResultsPanel.Pubmed_RVDocSum Effect of a mouthrinse containing selected propolis on 3-day dental plaque accumulation and polysaccharide formation.
 http://www.ncbi.nlm.nih.gov/sites/entrez?Db=pubmed&Cmd=ShowDetailView&TermToSearch=16054589&ordinalpos=1&itool=EntrezSystem2.PEntrez.Pubmed.Pubmed_ResultsPanel.Pubmed_RVDocSum The influence of a novel propolis on mutans streptococci biofilms and caries development in rats.
 http://www.ncbi.nlm.nih.gov/sites/entrez?Db=pubmed&Cmd=ShowDetailView&TermToSearch=9405742&ordinalpos=21&itool=EntrezSystem2.PEntrez.Pubmed.Pubmed_ResultsPanel.Pubmed_RVDocSum Antimicrobial activity of propolis on oral microorganisms
 http://www.ncbi.nlm.nih.gov/sites/entrez?Db=pubmed&Cmd=ShowDetailView&TermToSearch=17285269&ordinalpos=1&itool=EntrezSystem2.PEntrez.Pubmed.Pubmed_ResultsPanel.Pubmed_RVDocSum The effect of bee propolis on recurrent aphthous stomatitis: a pilot study.
 da Silva, F.B. 2004. Natural medicaments in endodontics - a comparative study of the anti-inflammatory action, Braz. Oral Res. 18,174-179.
 Gambichler T, Boms S and M Freitag 2004 Contact dermatitis and other skin conditions in instrumentalmusicians. BMC Dermatology 2004, 4:3
Tomás-Barberán FA, García-Viguera C, Vit-Oliviera C, Ferreres F & F Tomás-Lorente 1993. Phytochemical evidence for the botanical origin of tropical propolis from VenezuelaPhytochemistry 34, 191-196.
 http://links.jstor.org/sici?sici=0002-9122(198409)71:8%3C1149:TRORIA%3E2.0.CO;2-4 The Role of Resin in Angiosperm Pollination: Ecological and Chemical Considerations
 http://ecam.oxfordjournals.org/cgi/content/full/2/1/29 Recent trends and important developments in propolis research
 Montenegro G et al. 2004. Similitude pattern and botanical origin of the Chilean propolis. Phyton 145-154
 http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?cmd=Retrieve&db=PubMed&list_uids=1804186&dopt=Abstract Isolation and identification of antibiotic constituents of propolis from Henan.