Update, November 2017/: We no longer stock the twin beads, but the superduo range is now bigger than even and continuing to grow. The patterns below work well if you use all superduos.
Twin seed beads are small beads with two holes. There are two types, twin seed beads and super duo twin beads. Twin seed beads are made by extruding glass rods in to shape and cutting to size. Super Duos are pressed from molds individually. They are mostly the same size, but the superduos have a fatter middle section. They may be interchanagble in patterns, but it really does depend on the pattern.
Preciosa make the twin seed beads and have published several patterns to provide some inspiration.
Here are links to many of the patterns provided by Preciosa:
Twins are being run out, going forward we will be stock superduos only. We have an ever increasing range of colours.
Superduos can be found under Czech glass > 2+ hole beads