Gum arabic is a complicated mixture of long and short chains of sugars (arabinogalactan oligosaccharides and polysaccharides) and glycoproteins, (proteins with sugars attached).
It is a powder which becomes a sticky glue or gum when mixed with water.
It is extracted from the sap of the Acacia senegal and Acacia seyal trees native to the Sahara desert.
Gum arabic (gum acacia) is used as the lickable glue on stamps and envelopes, and in soft drinks as a flavor stabilizer (emulsifier). It has components which bind to water, and components that bind to oils, making it an emulsifier, but not a surfactant (because of the high molecular weight).
It is used in hard candies, and as a foam stabilizer in marshmallows.
The similar products gum tragacanth, gum ghatti, and gum karaya are used in much the same way, as acid resistant thickeners in sauces and salad dressings.