To begin with: I recommend you to buy this game, it's more then worth it.
Jedi Knight II: Jedi Outcast is a game in the series of Dark Forces and Jedi Knight. In my opinion it is the best game in the whole series (some may disagree) because it contains a very nice single player and and very good (and well balanced) multiplayer.
Singeplayer: The SP is set after the collapse of the empire. The ex-Jedi Knight Kyle Katarn is working as a mercenary for the new formed republic, trying to find outposts of the Remnant troops of the empire, who still work against the republic. In the process of the singleplayer Katarn regains his jedi power and discovers the real aims of the imperial troops. The story telling is very well embeded in the normal game flow, which is a good mix of action (fighting with weapons, lightsabers, force) and thinking (solving puzzles, finding ways). There is an option to set the level of difficulty, so the game is demanding, no matter how good or bad you are.
Multiplayer: After the SP (which is quite long), you may want to play MP. The general MP is very similiar to other Quake3-Games: You have Gametypes like Deathmatch, TD, CTF etc. But there are also some special gamemodes like JediMaster (one player is the jedimaster with every force power, the rest just owns weapons and has to kill him), CTY (like CTF but the flag-stealer isnt allowed to use force), Holocron (the force powers are spreaded at the maps in little boxes which you can pick up). After you joined a MP server you'll have to set your name, your skin and your force powers, where you can choose either the dark or light side and disturb force points in the different powers (like lightning, absorbation, healing, jump, speed). Note that there servers which disable weapons or forces, leaving you with nothing more than your saber ;)
The MP experience itself is unique - you can have normal deathmatches there, but also nice chats or exciting light saber duels; the lightsaber fight is the best I have ever seen in a Star Wars game: you can choose your style (fast, medium, slow means: less, normal, many damage per hit) and the saber reacts to every mouseclick you do, so with a bit of training you can do really nice combos. Also, back then, the modding community was quite active, giving you access to dozens of very good maps, tools and server mods.
Sooo all in all - buy the game, you'll not regret it :D
ps: I know Steam shows 0 hours on record, thats because my old JK wasnt linked with steam - my real play time lies anywhere among 500 and 1000 hours.
pss: ooh and sorry for my english, this isn't my main language.