Differences between canned and fresh tomatoes
Canned tomatoes, compared to fresh, are:


Cooked.  As part of the canning process, they are fully cooked.
Peeled.  Most canned tomatoes are peeled
Acidic.  For safety reasons, canned tomatoes often have additional citric acid added to the can (this prevents botulism growth)
Ripe.  The tomatoes to be canned are often vine ripened to a more ideal ripeness than retail tomatoes, and then immediately harvested, processed and canned.


Most of these attributes are actually an advantage in creating a pizza sauce, and so starting with a quality canned tomato may be far more convenient and flavorful than starting with raw tomatoes.