Do a search. There is a good link posted here not to long ago.

Here is a link that might be useful: suckers

I've always heard to prune the suckers...

Pruning isn't required in any way so you might also want to review the FAQ's here (linked at the top of the page) and make a note of the one called "To Prune or Not to Prune".

Dave

You asked what is a sucker, well as your tomato plant starts to put out limbs look at where it grew out from the main trunk of the plant and if there is a little growth showing up in that crotch it is a sucker, I usually cut off the first two and let the next two form limbs and cut off all above. OOPS up above I said limbs and meant leaves, sorry I knew better.

DAN

You asked what is a sucker, well as your tomato plant starts to put out limbs look at where it grew out from the main trunk of the plant and if there is a little growth showing up in that crotch it is a sucker, I usually cut off the first two and let the next two form limbs and cut off all above. OOPS up above I said limbs and meant leaves, sorry I knew better.

DAN

I don't know (ok, actually, I do), but there's one born every minute.

A squirrel and or a horn worm.
The first growth usually incapabale of formimg trusses, found at the intersection of a fully formed branch and the main stem.