How are they similar/different?
which one is better?
etc.
I had both Xs. Enjoyed x2 a lot. didn't have chance to play x3 cause of my stupid computer.
so how those games compare to freespace2?

The Freespace series is combat-focused. There is no trading and you can't fly a capital ship, there is no freedom of movement between systems etc. like there is in a game like X3 or Freelancer.
It has a great story, challenging enemies and good controls, though, and it's generally very frantic and fun. It's one of my favourite games, and in my mind the best space shooter ever.
I recorded a video to show what it's like while playing, I'll upload it to Youtube and edit in the link in a little while.
Edit: Here's the video.

Checkout the Battlestar Galactica: Beyond the Red Line demo [url=]http://www.game-warden.com/bsg/[/url].

Dude, equalize or augment forward shield! :p
Also, yeah, basically the only similarity between Freespace and the X2/3 is that they take part in space. X-series is very open - you can go where you want, do what you like, trade, fight, build, etc, but there's very little to do in terms of story. Freespace, on the other hand, is a mostly-linear progression of missions set along an unchangeable path - mission A leads to mission B leads to mission C etc. The content of some of those missions can vary slightly depending on how well you did in an earlier one, but they all boil down to a certain set of goals you need to achieve to progress along a set progression of the storyline. On the upside, Freespaces are a LOT more focused on the story {a good one at that, especially in F2} and much better combat mechanic.

Freespace 2 has a much more focused combat mechanic and a much faster pace than the X games.

Comparing Freespace and X is like comparing Crysis to Fallout... Yes, they are both played on a planet called Earth and there are people in either of them :D
X3 is like a Fallout in this comparison: Open space, you can play however you wish, you can both hunt pirates and fulfill optional quests you find along the way, you can fly around in your trade ship and ... Trade, or you can start a long tycoon-like game and build your own empire. You choose. It's pretty hardcore, not everyone figures what to do and they usually give up soon... Those who endure can find a fantastic game for a loooong time. It's a disadvantage, too: Playing X really IS like having a second job :D
Freespace is like a Crysis: You can't really choose WHAT you'll do, but you are sure to have a lots of fun during the play. Oh, and it has a great storyline.

X2 and X3 are sims, and that is what makes them great, and addictrive is a key word if you get into the game, the only real drawback was it was buggy as hell, I still have my boxes and disks but you can't uninstall it and reinstall it without have ISSUES, that said FreeSpace and FreeSpace 2 were a royal pain to play, and killing the capital ships was HARD, and I am playing FreeLancer now again for the upteem hundredth time, BTW X3-Terran Conflict is about to be released

Though the X series is constantly supported by Egosoft. Very few bugs exist in the newer patched versions.
If you have problems check out their tech support forum, maybe upgrade first if you haven't.
Terran Conflict is supposed to have multiple storyline viewpoints. The scale isn't quite as large as freespace though fairly close, esp when flying a tiny scout right next to a capital ship (that you can own ;D). Plus the X series has SETA, drop a station and turn time compression on over night. Do that a few times and you have cash for whatever you want while learning more about the game. Then when you're ready you can make a new game to see how fast income can be obtained with sparing uses of SETA.

Other posts here have covered the differences pretty well, so I'll just add this tidbit. Just before GOG opened up I picked up X3, as it looked very likely. Then I snagged Freespace here.
In practice, I am having a very hard time getting "into" X3. The story is good, and then it just leaves you hanging. Badly. I'm not even sure where to go sometimes, so I get stuck. Freespace, on the other hand, also has a strong story but is more directed. Much more. The missions come, and you WILL do them. Period.
I won't call either approach 'better.' In theory, I would love to play in an unbound universe with X3, but in practice I don't know what to do and for all the manual detail, they don't really give you a lot of hints. So X3 winds up being frustrating.
Freespace, on the other hand, doesn't have any options. Your single-player choice is 'continue campaign', which is continue the story. If the difficulty is ramped, some of the missions are pretty hard. Fail, and you go nowhere. There's nothing you can do about it, so that's frustrating.
But having said both, I find myself clicking on Freespace more. I know eventually I'll beat the missions, where with X3 it may be literally years before I guess what I was supposed to do.
For the price, get both.

Very different games. Freespace is space combat, X series is space trading / empire building.

Personaly, I find X3 unplayable without some kind of guide. So I'd advise you to get some of various game walktroughs etc. to find out what and where it is to do aroud X space. It may soud lame 'dah, I'm using walktrough!', but game gets much more entertaining... And eventualy, when you find out what various options it offers, even very revarding. Trust me. Feeling of new built space station is better than sex! (... All right, THAT was geeky :D )

Freespace doesn't have StarForce, and thus, wins by default.

Neither X2 nor X3 have had StarForce for some time. If you really want to avoid it, then get them via Steam. Failing that, install the game and patch it before the first run - the starforce drivers are installed when you run the game, not when its actually installed.

X2 and X3 are based on the old game Elite. It's about trading and scavenging and earning money to slowly upgrade your ship until you're ready do do some serious fighting. You've got a big open world you can travel around in and trade between.
The Freespace series is based off the X-Wing/Wing Commander games, combining the best parts of their respective interfaces. Taxpayers are paying for your stuff, so you can tear it up in space from the very beginning and never have to worry about finding the money to replace that missile you just shot at the enemy. Freedom to go anywhere, no matter how boring, is sacrificed for the best mission design in Space Sim history, with twists and turns and byzantine plotting and mission objectives that are constantly changing. Add the sheer heart-pounding adrenaline of zipping at full afterburners between behemoth capital ships carving into each other with beam weapons and batting you around with flak fire. But that's the tradeoff between the two series.
Oh yeah, the X games also let you found and manage your own exciting space corporation and play the space-stocks. Whoopee.

space stocks and upgrading my ship? I wasn't upgrading, but building my Great Fleet, called WTFleet! And then, destroying my enemies, meaning Everything That Moves! ... X games are more like Space Tycoons sometimes :D