Glycosides are molecules in which a sugar is bound to a non-carbohydrate moiety (functional group), usually a small organic molecule. The sugar group is known as a glycone and the non-sugar part is known as an aglycone or genin. If the glycone group is a glucose, then the resulting glycoside is known as a glucoside. If the glycone group is fructose, then the resulting glycoside is known as a fructoside.

There are loads of different kinds – for instance, salicin found in salicylic acid is a glycoside, rutin and quercetin are also glycosides, saponins are glucosides too – so I’ll refer you to this Wikipedia entry if you want to learn a lot more about them. 

They’re in so many water soluble extracts of plants, it’s hard to list them all here. A few of them are blueberry extract, yucca extract, Aloe vera extract, and St John’s wort hydrosol, amongst other ingredients, where they work as soothing ingredients.