Field placements in cricket are not standardised. There are several named field positions, and the fielding captain uses different combinations of them for tactical reasons. There are also further descriptive words to specify variations on the positions labelled by simple names, so that any position in which a fielder stands can be described.
The following diagram shows the rough positions of all of the simply named field positions. In this diagram, the pitch is indicated by three '#' marks; the striker's end is at the top. The bowler is not shown, but would be running upwards towards the bottom end of the pitch. The approximate field positions are marked with numbers or letters, according to the key on the right of the diagram. The three marks: '+', '*', and '~' indicate that the adjective shown at the bottom of the list can be used to describe a modification of that position, as shown in the example.
--------------------------------- 1 wicket keeper
/ \ 2 first slip
/ e h \ 3 second slip
/ \ 4 third slip
/ \ 5 gully +
/ \ 6 point +*~
/ \ 7 cover +
/ 2 j \ 8 extra cover +
| 43 1 d | 9 mid-off +*
| 5 | a mid-on +*
| 6 # i c | b mid-wicket +
| # | c square leg +~
| 7 # b | d leg slip
| 8 | e third man
| | f long off
\ 9 a / g long on
\ / h fine leg
\ / i bat-pad
\ / + deep (near boundary)
\ / * silly (near batsman)
\ f g / ~ backward (more 'up')
\ / eg.
--------------------------------- j deep backward square leg
(This picture will be replaced with an inline image when I have time.)
Other modifiers used to qualify positions:
square: close to a line perpendicular to the pitch, through the batsman;
fine: close to a line straight along the pitch;
short: close to the batsman.
The only restriction on field placements is that, at the time the ball is delivered, there must be no more than two fielders in the quadrant of the field backward of square leg. (This rule exists mainly for historical reasons - see the Bodyline section below.)
Sometimes fielders close to the bat wear helmets for safety. When not in use, the helmet (or any other loose equipment) may be placed on the field (usually behind the wicket-keeper, where it is unlikely to be hit by the ball). If any such loose fielding equipment is hit with the ball, five runs are scored, either to the batsman who hit the ball or as the appropriate form of byes. The ball is then considered dead and no further runs can be taken, nor can a batsman be run out.
If a fielder is wearing a protective helmet, and the striker hits the ball so that it bounces off the helmet, he may not be out caught off the rebound. If a ball rebounds from any other part of the body of a fielder, he may be out caught if another fielder (or the same one) then catches the ball before it hits the ground.