my two cents....
BF2 was designed to be played online, sure there is single player but i am sure you will find that the AI for the bots isnt all that good, eventualy you will venture online, then comes Multi-Player, you have a choice of ranked servers or non-ranked. ranked being that you work your way up the ranks, get badges, medals, and ribbons for various achievments. this can be both good and bad, at times it can be hard to find a good bunch of people to play with who are like minded, not a bunch of smacktards who are out to collect kills.
if you like to play single player then maybe you might want to try playing Coop, even thou EA doesnt support coop, there is a mod out there that is quite good, and the AI has been turned up, (no dumb bots there, very smart and cheeky) Coop is played online with other people who like the idea of teamwork and prefer to not play against people who do dumb things. either way it can be also interesting.
if you are interested in the coop mod, it can be downloaded Here
they also have a server that runs their mod
i find that a bit of both is good,