Seed beads
are uniformly shaped round or tube like beads. They come in a
variety of sizes with lots of different kinds and a lot of
different names. Most of the names are derived from the country
they we made in. Seed beads are most often used for Loom or off
loom bead weaving.
Types
Most of the
seed beads we see today in the US are made in the
Czech Republic or in Japan. The names of the beads usually
denote either where they came from or the company that made
them.
Czech (denotes the country it comes from)
Jablonex (may by
Jablonex comes from Czech Republic)
Matsuno (made by Matsuno comes from Japan)
Miyuki (made by Miyuki comes from Japan)
Toho (made by Toho comes from Japan)
Delica® (made by Miyuki comes from Japan)

1 Millimeter = 0.03937007874 Inches
My father had a professor's degree in math and taught math. Needless to say I am pretty good at math.
I did the long math and some of the math on the internet about beads is incorrect.
Let us not do the long complicated math and say that there are About 25mm's
to an inch.
Czech 11/0 beads that I carry are about 130 per gram and 20 by 12 per inch.
Japanese 11/0 beads that I carry are about 120 per gram and 17 by 12 per inch
Delica's are about 19 by 19 per inch
Well here is an example of Czech Seed beads. You see, beads are not square, well most seed beads aren't. So they can be taller than they are wide.
To make more since of it all, go to this page with pictures of some seed beads!