Elmer's Wood Glue (sometimes more formally called Elmer's Carpenter's Wood Glue) is designed mainly as an adhesive to bond wood to other wood. It's used widely as a construction adhesive and in industrial applications. Many home do-it-yourself enthusiasts also use the glue on a variety of projects. The active ingredients in Elmer's Wood Glue are primarily man-made chemicals. They include a proprietary blend of petroleum and natural gas along with other natural elements that are ubiquitous in nature.
Elmer's states that its wood glues aren't made from any animals or their hides; these types of materials are often a component of other traditional types of wood glue. Elmer's glue was a 20th century innovation that took advantage of the chemical formulation of polyvinyl acetate, or PVA. When combined with petroleum in a water-based solution, it can act as a powerful adhesive that can effectively bond wood.
Water is a primary constituent used in Elmer's Wood Glue to keep the PVA in a liquid state. When not mixed with water, PVA is a rubbery polymer. The polymers in the PVA are naturally sticky and give the glue it's adhesive qualities. This is helped along by the manufacturer's use of what it calls "tackifiers." These synthetic chemicals bind with the polymers to increase their stickiness.
Depending on the specific amount of PVA and petroleum as well as natural gas (which is usually injected into the glue), Elmer's Wood Glue may be white or yellow. In general, strong carpenter's glue adhesives are colored yellow because they're rendered more effective by higher levels of PVA and various tackifiers. Yellow, non-toxic coloring dyes are also added to differentiate between "professional grade" wood glues and common white glues used in everyday applications.
Most PVA-based wood glues, such as the kinds that end up in Elmer's products, aren't extremely water-resistant. If you need a wood glue that's impervious to water, you'll need to use one that makes use of epoxy or a brand that contains special cross-linked PVAs. Take care when using epoxies of any type, as they can be extremely hazardous and toxic. Standard carpenter's wood glues such as Elmer's, however, are rated non-toxic and are almost totally safe.