One of the central principles of golf is to play the ball as it lies. Therefore, if your ball at rest is moved, you must generally replace the ball. Whether or not a penalty applies depends on who moved the ball, how it was moved, and where on the course the movement happened. If the ball is moved by another player in stroke play or by anyone while searching for it, there is no penalty. However, when not searching, you are expected to be careful around your ball and will usually get a one-stroke penalty if you cause it to move. However, the putting green is a special case where there is no penalty for any accidental movement of a ball by the player or anyone else. In match play, if your ball is moved by your opponent (other than on the putting green or during search), the opponent will usually get a one-stroke penalty.
If your ball is moved by an outside influence, such as an animal or a spectator, there is no penalty to anyone and the ball must be replaced. If your ball is moved by a natural force, such as wind, water, or gravity, you will usually play the ball from its new location without penalty.
When your ball in motion is accidentally deflected or stopped, there is no penalty and you will normally play the ball from wherever it lies. This includes when your ball accidentally strikes your body or your equipment.