Monopole (what most speakers are) fire sound in 1 direction. Often called a "Direct Radiator" because they fire sound directly at the listener.
Bipolar speakers fire sound in 2 directions. Often forwards and backwards so you get a back-echo some short time later, but sometimes they fire 90 degrees to each other.
Dipolar - these also fire in 2 directions, but usually along a wall and not directly at the listener. They became popular when ProLogic video tapes emerged. The guys at Dolby wanted rear speakers with indistinct sounds like wind/rain/noise, but DID NOT want you to be able to tell where the speaker was. So they wanted a speaker that only bounced sounds off of a wall. These speakers have fallen out of popularity with DVD's and 5.1 systems where you WANT your attention pulled to the speaker location. This is sometimes called a "indirect speaker".
Tripolar - these are a bit of a gimmick and non-standard. They often have 2 drivers fireing along a wall like a dipolar, and a third driver that fires out towards the listener like a monopole. Hence "Tri" pole which means 3. Sometimes there is a small switch to change the speaker from indirect to direct.
Oh I understand the phase issue. I just did not want to throw too many concepts at Myo.
Myo: My advice is this -
Only consider direct-radiator speakers for your HT system.
If you hear some bipolar speakers like the DefTechs (which I own and love) and want them, keep this in mind: Bipolar speakers need 2-3 feet of space behind them to work well. If your room/spouse does not allow this, skip the bipolar speakers.