At this point, it shouldn't be a surprise that there are multiple endings for Mass Effect 3. What may surprise you is that to obtain the "best" ending in the game, you probably have to play multiplayer. The ending you get depends on your war assets and readiness level, which multiply to determine your effective military strength. This leaves a sour taste in my mouth for a "single player" experience.
You can get enough war assets through single player to obtain the ending, but it is very difficult. You basically have to complete every quest in the game. Essentially, the game forces you to play multiplayer co-op in order to complete your single player experience. This tactic seems a little underhanded. Maybe it's a way to drive the sales of unlockable multiplayer items. I don't know.