Plum Tomatoes don’t resemble plums in shape. They are more egg-shaped, if anything, though some people try to describe them as pear-shaped, which doesn’t help at all, because their shape is nothing like a pear, either.
Plum Tomatoes are more oval than they are round, and smaller as than “normal” tomatoes.” They are really good for making sauces with as they have less water in them, so you get a thickened sauce more quickly (mind you, this is really only true when you are comparing fresh plum tomatoes with fresh round tomatoes: it’s hard to know if you can see any appreciable difference with tinned.)
Because they are more meaty and less watery than round tomatoes, they are also really good at holding together on skewers on the barbeque.
Some say plum tomatoes are better for cooking than eating fresh, but many people also enjoy them raw.
Round tomatoes (though a bit more watery).
Same as for regular fresh or canned tomatoes.
We tend to think of plum tomatoes as an Italian thing, which they are in a way, as they are very popular in Italy, but given that they are also pretty widespread in Latin America, where tomatoes originated, they are probably actually a very old variety.