Here is the best I can steal from the USGA:
Definitions
All defined terms are in italics and are listed alphabetically in the Definitions section.
24-1. Movable Obstruction
A player may take relief, without penalty, from a movable obstruction as follows:
a. If the ball does not lie in or on the obstruction, the obstruction may be removed. If the ball moves, it must be replaced, and there is no penalty, provided that the movement of the ball is directly attributable to the removal of the obstruction. Otherwise, Rule 18-2a applies.
b. If the ball lies in or on the obstruction, the ball may be lifted and theobstruction removed. The ball must through the green or in a hazard be dropped, or on the putting green be placed, as near as possible to the spot directly under the place where the ball lay in or on the obstruction, but not nearer the hole.
The ball may be cleaned when lifted under this Rule.
When a ball is in motion, an obstruction that might influence the movement of the ball, other than equipment of any player or the flagstick when attended, removed or held up, must not be moved.
(Exerting influence on ball – see Rule 1-2)
Note: If a ball to be dropped or placed under this Rule is not immediately recoverable, another ball may be substituted.
24-2. Immovable Obstruction
a. Interference
Interference by an immovable obstruction occurs when a ball lies in or on theobstruction, or when the obstruction interferes with the player’s stance or the area of his intended swing. If the player’s ball lies on the putting green, interference also occurs if an immovable obstruction on the putting greenintervenes on his line of putt. Otherwise, intervention on the line of play is not, of itself, interference under this Rule.
b. Relief
Except when the ball is in a water hazard or a lateral water hazard, a player may take relief from interference by an immovable obstruction as follows:
(i)Through the Green: If the ball lies through the green, the player must lift the ball and drop it, without penalty, within one club-length of and not nearer thehole than the nearest point of relief. The nearest point of relief must not be in a hazard or on a putting green. When the ball is dropped within one club-length of the nearest point of relief, the ball must first strike a part of thecourse at a spot that avoids interference by the immovable obstruction and is not in a hazard and not on a putting green.
(ii)In a Bunker: If the ball is in a bunker, the player must lift the ball and drop it either:
(a) Without penalty, in accordance with Clause (i) above, except that thenearest point of relief must be in the bunker and the ball must be dropped in the bunker; or
(b) Under penalty of one stroke, outside the bunker keeping the point where the ball lay directly between the hole and the spot on which the ball is dropped, with no limit to how far behind the bunker the ball may be dropped.
(iii)On the Putting Green: If the ball lies on the putting green, the player must lift the ball and place it, without penalty, at the nearest point of relief that is not in a hazard. The nearest point of relief may be off the putting green.
(iv)On the Teeing Ground: If the ball lies on the teeing ground, the player must lift the ball and drop it, without penalty, in accordance with Clause (i) above.
The ball may be cleaned when lifted under this Rule.
(Ball rolling to a position where there is interference by the condition from which relief was taken – see Rule 20-2c(v))
Exception: A player may not take relief under this Rule if (a) interference by anything other than an immovable obstruction makes the stroke clearly impracticable or (b) interference by an immovable obstruction would occur only through use of a clearly unreasonable stroke or an unnecessarily abnormal stance, swing or direction of play.
Note 1: If a ball is in a water hazard (including a lateral water hazard), the player may not take relief from interference by an immovable obstruction. The player must play the ball as it lies or proceed under Rule 26-1.
Note 2: If a ball to be dropped or placed under this Rule is not immediately recoverable, another ball may be substituted.
Note 3: The Committee may make a Local Rule stating that the player must determine the nearest point of relief without crossing over, through or under the obstruction.
24-3. Ball In Obstruction Not Found
It is a question of fact whether a ball that has not been found after having been struck toward an obstruction is in the obstruction. In order to apply this Rule, it must be known or virtually certain that the ball is in the obstruction. In the absence of such knowledge or certainty, the player must proceed under Rule27-1.
a. Ball in Movable Obstruction Not Found
If it is known or virtually certain that a ball that has not been found is in a movable obstruction, the player may substitute another ball and take relief, without penalty, under this Rule. If he elects to do so, he must remove theobstruction and through the green or in a hazard drop a ball, or on theputting green place a ball, as near as possible to the spot directly under the place where the ball last crossed the outermost limits of the movableobstruction, but not nearer the hole.
b. Ball in Immovable Obstruction Not Found
If it is known or virtually certain that a ball that has not been found is in an immovable obstruction, the player may take relief under this Rule. If he elects to do so, the spot where the ball last crossed the outermost limits of theobstruction must be determined and, for the purpose of applying this Rule, the ball is deemed to lie at this spot and the player must proceed as follows:
(i)Through the Green: If the ball last crossed the outermost limits of the immovable obstruction at a spot through the green, the player maysubstitute another ball, without penalty, and take relief as prescribed in Rule24-2b(i).
(ii)In a Bunker: If the ball last crossed the outermost limits of the immovableobstruction at a spot in a bunker, the player may substitute another ball, without penalty, and take relief as prescribed in Rule 24-2b(ii).
(iii)In a Water Hazard (including a Lateral Water Hazard): If the ball last crossed the outermost limits of the immovable obstruction at a spot in awater hazard, the player is not entitled to relief without penalty. The player must proceed under Rule 26-1.
(iv)On the Putting Green: If the ball last crossed the outermost limits of the immovable obstruction at a spot on the putting green, the player maysubstitute another ball, without penalty, and take relief as prescribed in Rule24-2b(iii).
TT