You will find lots of different types of players in hold em. Loud, quiet, loose, tight etc though most players style of play can be categrosied into one of the following.
The Calling Station : You will encounter many calling stations when playing hold em. Calling stations want to be in the game as much as possible and see as many hands as they can to the river. They will rarely raise but will nearly always call a raise. Its great having a few calling stations at your table if you have a strong hand.
The Rock : Otherwise known as the tightass! People like this play very tight and will usually only go in on very strong pairs eg 2 kings etc. It is very unlikely that they you will lose a lot of your money to someone like this.
The Maniac : Nearly every table has a maniac. These types of players play very loose. They like to raise, they love to bluff and they want to be involved in as many hands as possible. You have to be careful when playing against a maniac as even maniacs get good hands occasionally. You will notice that these types of players have a high variance of chips - one minute theyve got a mountain of chips and the next minute they're down to their last few chips.
The Solid Player : This is the type of player that you should strive to be and the type of player that you should watch out. Solid players play tight but aggressive when they get involved in a hand. They dont get involved in a lot of pots but when they do they will raise and control the flow of the game.
What you have to remember is that nearly everyone switches their style of play during the game. A maniac who loses a two big hands in a row might tighten up his play. Conversely, a tight player who has just lost a big hand they were sure of winning might become a maniac for a while and bet in hands that they wouldnt usually bet on. The general concensus is that if your at a table with a lot of loose players (people who play a lot of hands) then you should play tight. If you are at a table where most players are tight then you should play more loose.