A glycoside (also spelled 'glykoside') is one of the three classes of poisons found in plants. (The others are alkaloids and aromatics). Chemically, a glycoside is one or more sugar groups terminating in some sort of amide group.
The poisons of glycosides work in various ways. One of the most famous is when the chemical is a cyanogenic glycoside, where the terminating amide group is a cyanide group. When the glycoside is digested, the cyanide group is freed and proceeds to poison the body.
Another variety of glycoside is the cardiac glycoside, which is a functional, not structural definition. A cardiac glycoside is any glycoside that causes poisoning by affecting the heart.
Some families of plants are well known for their glycoside content. All members of the rose family, for example, posseses cyanogenic glycosides to one degree or another.