Usually a jam contains as much sugar as it contains fruit. The two parts are then cooked together to form a gel.
In the European Union, there is the jam directive (Council Directive 79/693/EEC, 24 July 1979). It sets minimum standards for the amount of "fruit" in jam, but the definition of fruit was expanded. This was done to take several unusual kinds of jam made in the EU into account. For this purpose, "fruit" is considered to include fruits that are not usually treated as fruits, such as tomatoes; fruits that are not normally made into jams, such as melons and watermelons; and vegetables that are sometimes made into jams, such as: rhubarb (the edible part of the stalks), carrots, sweet potatoes, cucumbers, and pumpkins. This definition continues to apply in the new directive, Council Directive 2001/113/EC (20 December 2001).
Jelly is different from jam as it is made from mostly juice instead of fruit.
Fruit butters are made from slow cooking fruit to a smooth consistency.
As a gel, jam is neither a solid or a liquid. It can contain chunks of fruit which are solids. Once opened and out of the fridge the gel becomes more like a liquid, and is able to decay.