A cricket field is divided into a leg side; signifying the side towards the pads of the batsman on strike and an offside; which is the opposite side. Also, an area behind the square and in front of the square is defined. Ideally, a square is an area in the middle of the ground where main pitches are laid out. (as opposed to practice pitches which are, if present, usually laid out more towards the outfield). However, in the strict sense of the term, fielding positions behind square are the ones behind the stumps of a batsman on strike. Furthermore, the fielding positions can also be divided into the infield and the outfield, with the thirty yard circle taken as the reference point.
The offside infielders behind square are usually slips, gully, point while cover point, cover and mid-off are in front of the square . A lot of improvisations over the years have been carried out by captains leading to birth of new positions like the silly point for bad pad catches off spinners and short cover or short mid-off for a drive played too early by the batsman.
Infielders on the onside include leg slips (or slips), leg gully, square leg behind the square and short leg, mid wicket and mid on in front of square. Similar to the off side, straight mid on, just by the non striker has been used quite regularly esp. by the Aussies.
In the offside outfield, the fielders behind square are third man and a deep point. Sometime, two third man are employed by captains. In front of square, deep cover, sweeper cover, deep mid-off are the usual positions.
while on the on-side outfield, behind square, a fine leg, a deep backward square leg are the usual positions. while in front of square the positions are deep midwicket, long leg and deep mid on.
There is no hard and fast rule for placement of fielders, a lot a factors govern the decision on the part of a skipper viz. the batsman, the type of bowling, the condition of the pitch or the prevailing conditions, the match situation, etc.
Crowding the batsman is used as an intimidating tactic...who can forget the pictures of Dennis Lillee bowling with nine slips!!! or Shane Warne bowling to the English batsmen down under.
NB: no fielder is allowed to be stationed behind the bowlers' arm or the wicket keeper.