Just learnt these things a few days back, when my friend bought a pair of paradigm dipole spkrs!
Bipole spks have more than one or 2 speakers on both sides, so sound can be heard in both directions near the rear wall. Both sides of the bipole speakers word simultaneously, which distinguishes it from Dipoles in which alternate speakers work and thus creating a diffuse surround sound effect. I have heard both are good when used as rears.
Bipole - 2 sides work simultaneously, creating diffuse sound effect, making the speakers virtually absent, meaning you can discern the direction of the sound coming from.
Dipole - one side works and the other side does not, vice versa, creating diffuse, enveloping sound effect. Highly recommended for movie freaks!