Definition from Big Bead Little Bead
"Seed beads are small uniformly shaped beads that come in a myriad of shapes, sizes, colours and finishes. Aside from embroidery, they are most commonly used for loom and off-loom bead weaving techniques such as brick stitch and peyote stitch. They can also be used for beaded flower making, simple stringing, and as spacers between other beads in jewellery making.
Glass is the most common material used to make seed beads, but historically they have also been made from metal or ceramic. Most modern high quality seed beads are made in Japan or the Czech Republic. The Japanese beads are manufactured by Matsuno, the oldest of the three Japanese seed bead manufacturers, in Osaka, Toho in Hiroshima, and Miyuki in Fukuyama, Hiroshima. These Far Eastern manufacturers have a well deserved reputation for achieving greater uniformity and consistency of size, shape and finish over Preciosa, their main Czech Republic based competitor, but their pricing does reflect this."
"The largest size of a seed bead is 1° ("one-aught," sometimes written 1/0) and the smallest is 24°, about the size of a grain of sand. However, seed beads that are size 5° or 6° are usually called "pony beads" rather than "seed beads"; the next larger class of seed beads, from 3° to 4°, are usually called "trade beads"; the largest class of seed beads, including 1°, 2°, and anything larger, are usually referred to as "crow beads."
Most modern seed bead work is done using seed beads ranging in sizes 6°, 8°, 11°, 12°, 13° and 15°.... The extremely small class of seed beads smaller than 15° have not been in production since the 1890s and any in existence are usually considered antiques."
Bugle beads, which are thin tube beads of various lengths, are often classified as a type of seed bead (I think because the width/hole size is very similar to an 11/0 seed). Likewise, some other shapes, such as cubes, or even the popular SuperDuo two-holed beads are sometimes called seed beads. In my mind, though, these are separate varieties of beads, not types of seed beads. I'm kind of a purist, I suppose.