A player's aggression or passivity refer to how he plays a hand. Aggressive players are usually betting and raising while passive players tend to check and call. When a player who exhibits one style suddenly switches to a different style, look out. This can indicate she's holding a monster.
The looseness or tightness of a player refers to a players hand selection and how many hands he plays overall. A loose player will play more hands and show down some marginal cards. A tight player tends to stick to premium hands, thus playing few hands.
Players who "see monsters" where there aren't any are called weak. They tend to put their opponents on a single hand rather than a range of hands, and that hand is usually a big one. They will make "great laydowns" and take pride in them. Weak players are easily bullied by aggressive opponents.