There have to date been three editions of Space Hulk. Here's the skivvy on number one.
First of all came first edition (gasp!). In this progenitor of two decades of awesomeness there are many of the same rules that are present in the current edition. The Space Marine player has a set of objectives to accomplish, and must generally do so in the face of a swarm of unending alien attackers (think the movie Aliens).
The game board is constructed out of smaller tiles representing rooms, corridors and junctions. These are arranged differently according to the mission being played.
Models have Action Points that they expend preforming actions such as moving, turning and attacking. Space Marine terminators have four, and their opfor, the deadly alien genestealers, have six. Genestealers have a semi-hidden movement when they enter the board in the form of "blip" tokens, representing an unknown number of contacts on the marines' radar, and ranging from one to three actual models when converted.
Weapons fire is fairly simple: only marines have firearms in the basic game. Regular marines and the squad leader are armed with Storm Bolters. For the cost of one action point, they may fire at any genestealer within line-of-sight and their arc of fire. Then, the marine player rolls two dice. If either dice is a six, the genestealer is destroyed. In the event that a marine misses his first shot, he may fire again with a "sustained fire" bonus, this time needing a five on either dice. This bonus stacks down to a 3+ to kill. One marine per squad is normally armed with a heavy flamer. This weapon has a range of only 12 squares, and is limited to 6 shots, but it affects an entire board section when fired and kills all models on a roll of 2+. It is also effective as a denial weapon, as a flamed section blocks line of sight and movement until the marine player's next turn. Line of sight is essentially determined from center of occupied square to center of target square.
Close combat is equally straightforward. For one action point a model may attack a model directly in front of it in an adjacent square. A marine rolls one die, and a genestealer rolls three. Whichever has the single highest die roll wins the combat, and the looser is eliminated. A terminator sergeant is more skilled than a regular marine, and so gets a +1 to this die roll.
Missions are fairly straightforward but allow for versatility. Get X number of marines from point A to point B, kill Y number of genestealers before kicking the bucket, of the famous go flame room Z from mission 1.
Basic First Edition is great. It's simple, straightforward, fast-playing and intuitive, not to mention tense! Even if you choose to not play with a timer for the marine player (which really adds to the claustrophobia he feels while playing) almost every game is a close one.
Using nothing but the core rules is somewhat limiting. While you can make your own scenarios, the game only comes with 6 official missions. This gives you a fair deal of replay value, especially switching sides, but does get old eventually. Also, the marines are limited to just a few troop types, and the stealers even more so.
Deathwing and Genestealer expansions, as well as second and third editions, to come later.
(insert pithy exiting comment here)