Well, now that there is an expansion there's more to talk about - and basically two almost seperate games to play.
First of all, BSG
Two teams, 3-6 players. Allegiances are distributed randomly (by cards) and you're not allowed to reveal that (with some caveats of course). The humans hope to get to a new home by travelling a certain distance (a number of jumps, which varies depending on the destinations chosen). However, they suffer from a limited supply of ships, food, fuel, population, and morale. The cylons win by eliminating at least any one of the four main supplies. All the while, crises continue to occur with much be dealt with (in the form of skill checks passed by colored cards of different values put secretly into a pile) - any players can help or hinder at will.
While not the most eloquent description, it provides a bit of mechanical / game play over view. I have played countless hours with my friends since before it was released at pax last year -- probably the best board that we own collectively. It often involves good luck, strategic thinking, and a _huge_ amount of bluffing, lying, and screaming!
The Expansion Pack (I've forgotten it's name).
Introduces new cards, new player types, a new board, new characters, and some new objectives. The humans travel to New Caprica, which is a shorter distance away - but once there, they must survive until the fleet can return to rescue them. During the course of this game, the cylons can have a slightly more involved role after they are revealed (not to say that they didn't before). My friends and I often play with 7 players because of the addition of the new "Cylon Leader". There is one cylon leader (optionally), and they begin play revealed. Instead of belonging to one team or the other they can have different objectives which need to be completed in order for them to win the game - this can either help or hinder the humans. Example: Humans must survive, but have less than 2 food or morale. Other objectives include the cylons wining with some additional clauses. I find that in general the expansion improves the game (and has snazzy moulded plastic basestars!).
Example of Gameplay (once characters are chosen etc.)
Player 1:
1. draws 5 cards of appropriate colors from the skill decks
2. may perform a move and an action (play a card, activate a tile)
3. Ends turn by drawing a crisis card and resolving it
a. crisis involve skill checks that must be beaten by players putting in skill cards
different colors can help or hurt depending on the crisis at end
most crisis have results that are "bad" for the humans, but there are
beneficial results as well
Play continues on to next player
Obviously there's alot more than I can describe in a quick post, but hopefully that helps. It is fun, and can be easy or challenging depending on how apt the people you play with are at lying and concealing their intentions. However, basically everyone I have played with enjoys the game (even casual/non-gamers) and can become quite good at it.