The difference between jams and preserves is how the fruit is prepared. Preserves contain whole fruit or coarsely chopped fruit depending on the size of the produce used. Jams contain mashed or even finely chopped fruit. It is a falsehood that preserves do not contain pectin. Both jams and preserves will contain pectin or the recipe will use some other thickening trick.
How the fruit used in fruit preserves is handled depends on the size of the fruit. When making preserves, small pieces of fruit, such as blueberries, would be used whole. Medium size pieces of fruit, such as strawberries would be cut in half or in quarters. Large pieces of fruit, such as apples would be coarsely chopped.
On the other hand, to prepare soft fruit for jams, the fruit is typically placed in a bowl and crushed into a pulpy substance with a potato masher. Small chunks of the fruit will remain, but the size of the fruit pieces will be very small. Soft fruit can also be cut into small chucks, but this takes more time and the end result is basically the same as mashing. Hard fruit used in jams, such as apples, is prepared by finely chopping the fruit.
The difference between jams and preserves is easily seen when both are spread out on a slice of bread. For example, strawberry jam will spread very easily and will streak the bread with a strawberry jelly and small pieces of fruit. Strawberry preserves will be harder to spread because suspended in the sugar gel will be larger, chunky pieces of fruit which will impede the spreading. Both jams and preserves, however, will taste about the same. Some canners feel that preserves retain the fresh fruit taste better than jams.
Pectin is a substance naturally found in fruit and also added, when necessary, to fruit spreads to thicken the condiment. Some canners believe another difference between jams and preserves is that jam recipes use pectin when the fruit does not naturally contain enough pectin and preserves use some other method to thicken the spread when necessary.
In fact, pectin can be added to preserves when required without needing to change the name of the spread to jam. Also, jams can use alternative thickening methods and do not have to include pectin. Preserves and jams can be thickened by including some slightly unripe fruit along with ripe fruit. Unripe fruit has a higher concentration of pectin than ripe fruit. These condiments can also be thickened by adding lemon, along with the sugar, to encourage the natural pectin to leach from the fruit.