A flat pitch would have very few hints of grass or cracks on it and would yield relatively even bounce. The ball would come on to the bat pretty nicely, making the track easy to score off.
When there is moisture in the pitch, it makes life difficult for batsmen because the ball doesn't come on to the bat as smoothly as on a track with lower moisture content. As the day goes on, however, the moisture may evaporate, thereby making batting easier. On a fourth day track, it is likely that cracks will form on the surface as the pitch dries out further, enabling spinners to wreak havoc.
When a pitch stays low (e.g. in subcontinental dust bowls), batting once again becomes difficult and bowlers often reap the benefit provided by this phenomenon by bowling wicket to wicket in the hope of getting LBW's in their favor.
A grassy pitch, on the other hand, facilitates fast bowling by offering steep bounce to pacers.
In an international match (or any official match with an official team of umpires), bowlers will be warned if they fail to stray away from the center of the pitch in their follow through as their boots may cause damage to the pitch. Batsmen will get the same treatment for running right through the center.
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Screw the IPL, I'm going to the MLC!
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