Sheryl Rosen wrote:

> Which begs the question: What exactly are soda crackers?

A name first used in N. America, from the mid-18th
century onwards, for a plain, unsweetened, dry,
hard, bread product; thus corresponding to part
of the domain covered by the wider English term
"biscuit." When crackers are broken into pieces
they make a cracking noise, which accounts for the
name. Crackers may be leavened or unleavened.
Those of the former sort were formerly baked by a
particular method which called for a dough leavened
with bicarbonate of soda (hence the term "soda
cracker") and left to stand until pockets of carbon
dioxide formed in the mixture. When biscuits of
this dough were placed in a very hot oven they
rose quickly, giving the characteristic texture.
Unleavened crackers may be made from flour and
water only (as are matzos) or with the addition of
a little salt. Some examples of this sort are the small
oyster crackers, used on top of seafood
chowders, and the crackers know as ship's biscuit...
The cracker barrel was an institution in
American general stores and groceries which sold
crackers loose in bulk. The term was first used in
print in the 1870s..."
---The Oxford Companion to Food, Alan Davidson
[Oxford University Press:Oxford] 1999 (p. 222)

> And how do they differ from saltines?

"In certain areas of the midwest there was a strong
preference for a cracker called "Premium Flake"
or "Saltina," which replaced Uneeda in popularity.
The Premium Saltine, a soda cracker, had long
been a favorite in certain areas of the midwest,
particularly in the Missouri area where it was
invented. It had won prizes as far back as 1876.
[Thomas L.] Green was jealous for his favorite
soda cracker, Uneeda Biscuit, and kept the
Premium Saltine confined to its home ground. However,
in the twenties the Saltine began to be manufactured
and distributed in factories throughout the
country and was soon in wide demand. The popularity
of the Saltine was increased many times over
in the years ahead."
------Out of the Cracker Barrel: From Animal
Crackers to ZuZu's, William Cahn [Simon & Schuster:
New York] 1969 (p. 198)

Quoted from:
http://www.gti.net/mocolib1/kid/foodcookies.html