There are many types of players you will find in real world, paid and free online poker. I will write about the main ones, how to recognise them and adjust your poker game to play against them.

Rocks - players who play very few pots and like to fold.
This type of poker player is one who tend to only play strong hands. They will avoid playing any weak hands and will fold these. If they stay in the hand and don’t fold then its best that you fold as they very likely have a better hand than yourself. However they are very good opponents to steal the blinds from

TAGs - tight before the flop and aggressive after the flop.
This is considered by many the most profitable style of play. I agree and these kind of players are best avoided but you wont find many of them on free online poker or low stakes games because it takes quite a lot of discipline and practice.

Maniacs - crazy players who play very aggressively.
This type of player is one who tends to bet and raise a lot and wont check. They are difficult players to play against, but if they are after you and you have a strong hand.

Calling Stations playing poker
This type of play is rarely profitable.
It has been proven by pros that it’s better to bet and raise then check and fold.
Against these players you should not try to bluff them because they will often call with poor hands. However you should bet when you have a good hand because they will call with trash hands pretty often. If they bet then you should very often fold because they rarely bet so they must have something strong.

To identify in which category the players you play against are in you should look for signs that match the short description on each type. It will usually take a bit of time before you can tell but once you can you will get a lot more success against them. These are not the only types of player you will find, look out for more articles covering the other types and styles of poker player you may find playing free online poker.