The differences can be found here:
en.wikipedia.org/wiki/Fruit_preservesGenerally, jams are pulpy, whereas jelly has the pulp removed. That's the main difference.
Jam contains both fruit juice and pieces of the fruit's (or vegetable's) flesh, however some cookbooks define jam as cooked and gelled fruit (or vegetable) purees. Properly, the term jam refers to a product made with whole fruit, cut into pieces or crushed. The fruit is heated with water and sugar to activate the pectin in the fruit.
Jelly is a clear or translucent fruit spread made from sweetened fruit (or vegetable) juice and set using naturally occurring pectin. Additional pectin may be added where the original fruit does not supply enough, for example with grapes. Jelly can be made from sweet, savory or hot ingredients. It is made by a process similar to that used for making jam, with the additional step of filtering out the fruit pulp after the initial heating.
And of course, gelatin desserts are known as jello in America and Canada and jelly elsewhere:
en.wikipedia.org/wiki/Gelatin_dessert* In many of the Commonwealth Nations, gelatin desserts are called jelly.
* In the United States and Canada, gelatin desserts are called jello (a generic name based on the brand name Jell-O) or gelatin.
