I haven't played for a while, so some new variations on the rules may make what I say outdated but...
Space Marines: Basic troops are great at everything compared to everybody elses rank-and-file, and as a whole they have good armour, good weapons. That balances out (somewhat... they are GW's bread and butter, so they do tend to get a little over powered occasionally) by only being able to field comparitively small armies. Lack a little in the vehicle department.
Chaos Space Marines: As above, but evil. Rulewise they have a little more flexibility (depending on the codex) but they don't get as much love from GW. Still, they also get demons that worship the Chaos gods. Just not Chaos Demons - that's a different army now.
Imperial Guard: their armour is made of cling wrap and their guns are just laser pointers with scopes taped to the top, but you fire off about a million shots a turn, so you'll kill something. Also, they have arguably the best tanks in the game.
Orks: orks is for fightin'. They don't have a lot of armour but they are tough, and they like to get in close for the hand to hand kill. Their greatest stength is numbers.
Tyranids: like the zerg, except... uh... well yeah, just like the zerg. Eat everything, everywhere. Have numbers and speed in a big way, and nasty in close combat, but even orks will outshoot them. A few big monsters flesh them out since they dont have vehicles. Carnifexes are fun.
Tau: hi tech shooty army. High mobility battlesuits, hover tanks with railguns... Not very good at handling close combat though, and unfortunately that is a pretty big weakness in a galaxy where even the tank commanders want to melee.
Eldar: space elves in spaaace. Fast, lightly armoured, specialists. Every unit has a very specific role that they really really excel at, but they don't do so great when you need them to cover for someone else. When the plan comes together they work like a well oiled machine, but it can sometimes be easy to have the whole plan fall through when you lose that one crucial unit. Have arguably the bext psychic users in the game.
Dark Eldar: evil space elves. Remember I said the eldar were fast? Yeah. These guys are the fastest. Close combat oriented, very light armor but if they survive long enough to reach combat, they strike hard and fast before the enemy can react.
Chaos Daemons: suffer from a severe lack of shooting ability, but oh my god do they love hand to hand. I mean, what did you expect. They're demons. Every unit has an invulnerable save, which is good. Only like 2 units also have an armor save, which is bad.
Necrons: Robot zombies! Pretty tough, have nasty weapons that laugh at tanks. Don't have a lot of numbers though, and only the one vehicle (though it is BADASS). Oh, and did I mention that they resurrect themselves on the battlefield?
My god was that big. I haven't even done the Demonhunters or SoB yet. They are about to rerelease the demonhunters, but I don't really known anything about them beyond they are like space marines cranked up to 11, with even LESS troops, whereas sisters of battle are imp guard in heavy armour with a fire fetish. Personally I play Nids and Chaos, but thats mainly a character thing. My advice is to read up on the background, find a race you like, play a few trial games at your local with them, repeat til you find an army you like. It is far too expensive a hobby for you to start an army, change your mind then start another.
