The line of putt must not be touched, except to remove loose impediments, lifting or replacing the ball, repairing a ball mark (old or your own) or removing movable obstructions. The player can place the club down in front of the ball when addressing, but may not press anything down.
The most common breach of this rule occurs when a player, partner or fellow competitor physically touches the point on the green where they should be aiming a putt. The line of putt can be pointed to, just not physically touched.
Penalty for breach of rule: for stroke play, a two-stroke penalty; for match play, loss of hole.
A player, partner or caddy can indicate a line for putting prior to but not during the stroke. In doing so, the line of putt can not be physically touched.
This means you cannot have your buddy stand behind the hole so you can aim at his foot. Placing the flagstick on the ground behind the hole also could be construed as a rule violation. It is better to place the flagstick off the side of the green.
Penalty for breach of rule: stroke play, two-stroke penalty; match play, loss of hole.
If a player's ball moves after it is addressed, other than as a result of a stroke, the player is deemed to have moved the ball and incurs a penalty of one stroke, and the ball must be replaced, unless the movement occurs after the backward movement of the club and the stroke is completed.
Usually this happens in three situations. When grounding the putter to align the putt, it is easy to accidentally bump the ball. A strong wind could move the ball. If the green is very fast and your ball is on a mound, the ball could move. As long as you are aware of the situation, you may be able to avoid a penalty. If the ball moves as you start your backswing, don't stop! Completing the stroke cancels the penalty.
Penalty for breach of rule: stroke play, one stroke penalty (and ball must be replaced to original position); match play, loss of hole.
If a ball or ball-mark is accidentally moved in the process of lifting the ball under a rule or marking its position, the ball or marker must be replaced. There is no penalty.
This rule is self-explanatory. Many golfers are under false assumptions that there should be a penalty.