Jelly is a sweet or savoury food gel, usually made through the addition of gelatin or pectin. Sweet food gels include gelatin desserts such as Jell-O and blancmange or fruit jam. Savoury food gels include aspic or plain gelatine. Vegetarians & Vegans make jelly not with gelatine (which comes from cows), but with agar (which comes from seaweed) which comes as crystals into which you pour water.
In beekeeping, royal jelly is a specific foodstuff made by bees which, when fed to bee larvae, turns them into queen bees.
Regional variations
In North America, the term is often used to mean jam in general, whether it has gelled or not. Concord grape jelly is usually used in the staple of North American school lunches - the peanut butter and jelly sandwich (PBJ or PB&J). Mayhaw jelly is a delicacy in the American South.
Jelly babies are a popular gelatine based confectionery in the UK and gummi bears are popular worldwide.