But there's more to it than that, isn't there? Where do the other things fit in and what are they (e.g. an OVA, a manga, a novel etc)?
Yeah, there's a
lot more to it than that.
First of all, the OVA you're thinking of is probably .hack//Liminality, which comes with each of the four games and documents events taking place in the real world which relate to the events of the games. Like the games, Liminality comes in four parts.
The manga you're thinking of is .hack//Legend of the Twilight, which is really more like an epilogue to a series which "doesn't need an ending". There's also another series called .hack//XXXX, which is a manga adaptation of the events of the four games. There's a lot in here that violates canon (for instance, Cubia takes the form of another player-character instead of the giant skull-with-roots he usually looks like), so this is really more of an alternate universe.
Legend of the Twilight is also an anime series, usually called .hack//Dusk for some reason (probably just because it's shorter to type), but it doesn't have a whole lot in common with the manga.
.hack//AI Buster, a novel I have in my possession, tells a story about a player called Albireo and his interactions with Aura's prototype, Lycoris. This takes place prior to SIGN, and part of the story documents Balmung and Orca's infamous battle with The One Sin. AI Buster 2 is a series of short stories that cross over with several different parts of the series, including SIGN and Dusk.
There's also .hack//Another Birth, four novels which retell the story of the games from Black Rose's point of view. These are actually pretty good, since Black Rose is often off on her own during the events of the games, and you get to see what she was up to. You also find out a lot more about her character than what is revealed in the games, since the story follows her in the real world as well.
Then comes Roots and GU. If you picked up the special edition of the first game like I did, you get a bonus "Terminal Disk" which has unlockable content that reveals some of the events that led up to GU, including the creation of the Avatars from the remains of the Eight Phases. It also reveals a few fun facts for those who don't catch on to events in GU; Haseo is the same player who played Sora in .hack//SIGN, for instance.
Finally, there's the .hack//Trilogy OVAs mentioned above, which seem to be a retelling of GU with a lot of differences. Time will tell whether this is any good or not, but I'm putting my money on a smash hit.
I'm sure there's more that I'm forgetting, but that's everything I can think of at the moment. Don't let the massive amount of content intimidate you; just play the first four games through, then add to your knowledge base with the rest of the series a bit at a time. Or just visit
.hack's Wikia Page
*EDIT*
Then again, if you got bored just playing the first game, you might be better off just looking for a copy of .hack//Rebirth with the Terminal Disk (or watching the videos on Youtube; just search for "terminal disk"); the next three games don't add too much to the gameplay, and GU is
way better in every way than the first quartet.