What does ohm mean? What's the difference between a 6 ohm speaker and an 8 ohm speaker? Does this have something to do w/ its efficiency? Is it better to have higher or lower ohm rating?

Ohms is not a measurement of efficiency but of impedance -- two 8-ohm speakers can have different efficiency ratings. Briefly, an amplifier has a recommended ohm value, and you should always choose a speaker with the same ohm value. If an 8-ohm speaker is recommended, connecting an 8-ohm speaker will give you the optimal power delivery to the speaker. Connecting a 4-ohm speaker, however, can result in worse performance and can (read: will) damage the amplifier because of the excess current drawn by the speaker. Connecting a 16-ohm speaker will also not be optimal (but in general will not permanently damage the amplifier since less current is drawn by the speaker). You can probably get away with using a low-ohm speaker in moderation, but running the amplifier long and hard can burn it out. Using the correct speaker will ensure that your amplifier/receiver is operating within it's intended limits.

As a side note, if you do have a mismatch but still want to use the speakers, rewiring several speakers in parallel or in series can change the impedance to the desired ohms.

I don't know that a 4-ohm speaker is 'better' than an 8-ohm speaker for HT, but it is generally desired to have a lower-ohm system. Ohm has little to do with the quality of the sound, but many speakers have lower ohms because of the increases sensitivity and apparent efficiency. Of course, like many other areas of HT, the numbers can be fudged, so never rely on low impedance as a sign of quality.

An ohm is a measure of resistance. So a speaker with a rating of 8ohms will have more resistance then a speaker with 6ohms. Most home theater speakers are rated at 8ohms where as in cars most of the speakers are rated at 4ohms. It all depends on what kind of load you are trying to give you amp.