"Animaux" pretty much covered it -- but here's my additional 2 cents -

Heirlooms are also known for their flavor -- wider variety of flavor and
usually better and more intense. They often will come back the next year as
"volunteer" plants from tomatoes that fell off and rotten into the soil --
seeding themselves for the next year.

However Heirlooms are generally not as hardy as the Hybrids which are
genetically bred and sometimes enhanced (I've heard of some of the varieties
bred for northern climates so they are more tolerant of freezes have been
genetically engineered to do that by introducing a chromosome from fish).
The Hybrids are bred for disease resistance as well -- so for instance if
you discover your soil has been infected with Tobacco Mosaic Virus, which
very adversely affects tomatoes as well, you pretty much have to find a
Hybrid bred to resist that or forget it!

There are literally hundreds of varieties and I used to grow most of mine as
Heirlooms from seeds I ordered, mostly because local stores weren't into
anything but the standard Hybrids, but in recent years lots of local stores
have caught on that there are a lot of people like me (and apparently
"Animaux") who like to try different varieties including Heirlooms and there
is a lot more choice. This year in just a regular old backyard garden I
have 14 tomato plants -- three are Heirloom I grew from seed, two new
Heirloom varieties I hadn't tried before but bought at Red Barn, and the
other nine are Hybrid varieties -- several I know I can count on and like
and a few new ones to try out. -- If you want one to stand Texas heat my
favorite is a Hybrid (several generations of improvement -- improved from
Heat Wave) named SunMaster -- it is what they grow commercially here, 4 to 6
ounces (not real big but the big ones start to "cook" on the vine). I
talked with a neighbor who said she had luck with one called Creole
developed in Louisiana.

One last thing -- even though Heirlooms will bred "true" -- that is true if
you are only planting one variety otherwise they will self improve
themselves and cross pollinate, so you have to just choose one or start from
seed if you want to be very sure of what you get. Seed seems cheaper but it
isn't that cheap anymore -- a packet contains less and less seed and it is
not all viable, not all will germinate let alone make it into a plant that
will survive -- realistically unless you want to plant all of one variety,
buying seedlings turns out cheaper in my experience.

--
Marta
(if you email me directly you need to remove the X )