Originally posted by: Garou24
Much of the Story and playstyle of the army is defined by the Background in the Codex (Army Rulebook). Thats were you will get a sense of the playstyle by reading th army specific rules and looking over the units.
The Tyranid codex your friend loaned you is an example of a zerg or swarm type of army. It has many low point models that you can use to overwhelm your opponent and provide cover for your nastier units that are following behind them.
One I dea would be to go to a local game shop and read through some of the codex there. Also you can look online and at wiki sites or check out the GamesWorkshop website for more information.
A quick discription for the armies that I can think of off the top of my head:
Space Marines - Good Meduim range Fire power, some solid close combat units and good mobility with Transports.
Chaos Space Marines - Good fire power, many close combat units, transports and tanks, but less options than Space Marines in some catagories.
Chaos Daemons - Many good assult units and some decent firepower, large monsterous daemons and quick movement by teleporting on to the game board instead of deploying like normal forces.
Eldar - Agile army with quick movement. a good mix of close combat and ranged fire power, their real strenght is their fast transports and tanks and other units.
Dark Eldar - Good Close combat army which relies on its speed to hit the enemy first. Fast transports but many units not able to stay in a long fight that drags out.
Imperial Guard - Excellent Long range bombardments and many many tanks. Low cost troops provide a large number of models on the board.
Necrons - Good short range to meduim range firepower, has the ability to repair models to save them from dying. Lacking many options for more troops.
Tyranids - Great close combat units with many low cost models. Some ok firepower on the larger models, have many big monster models.
Tau - Good long range firepower with high stranght guns and mobile units. Fast tanks and transports help them stay mobile.
Orks - Good close combat army with many low cost models, lacks useful shooting but makes up for it in amount of models you can use and quick movement.