Follow the link. There you'll find links to each of the games in the X Universe, where you'll find a brief synopsis of the plots.
--------------------------------------------------------------------------- "Remember the good old days when you could just slap Omni-Gel on everything?" - Shepard, ME2
"Not right now. Trying to determine how Scale Itch got aboard. Sexually transmitted disease carried only by Varren..... Implications unpleasant." - Mordin, ME2
Starting a custom game will throw you out into the universe as a random nobody, where you can do what you like, including starting each of the storylines once you get enough rep with the particular factions.
Starting one of the others will begin the game at the beginning of one of the storylines, such as Terran Defender will start the Terran storyline immediately, etc.
--------------------------------------------------------------------------- "Remember the good old days when you could just slap Omni-Gel on everything?" - Shepard, ME2
"Not right now. Trying to determine how Scale Itch got aboard. Sexually transmitted disease carried only by Varren..... Implications unpleasant." - Mordin, ME2