An obstruction is any artificial object except for integral objects and boundary objects.

Examples of obstructions:

• Artificially surfaced roads and paths, including their artificial borders.
• Buildings and rain shelters.
• Sprinkler heads, drains and irrigation or control boxes.
• Stakes, walls, railings and fences (but not when they are boundary objects that define or show the boundary edge of the course).
• Golf carts, mowers, cars and other vehicles.
• Waste containers, signposts and benches, and
• Player equipment, flagsticks and rakes.

You get free relief when the condition interferes with your lie, stance, or area of intended swing but not when it only interferes with your line of play.  To take free relief, you first have to find the nearest point of complete relief, which is the spot on the course nearest to where your ball lies, but not nearer the hole, where you could play your next stroke with no interference from the condition, you’re taking relief from. Then drop a ball within one club-length of that point not nearer the hole.
Exceptions:
Interference from Boundary Objects

Q. I have interference from an out of bounds fence, stake or wall – what are my options?

A. You do not get free relief from objects that define or mark the course boundary. You may not move objects marking course boundaries or take free relief from them like you would from other artificial objects, like a cart path, a building, or a stake marking a penalty area. Your options are to play your ball as it lies, proceed under penalty of stroke and distance by playing again from the spot of your last stroke (see Rule 18.1), or decide your ball is unplayable (see Rule 19.1)
Relief from Immovable Obstruction When Ball in Penalty Area

Q. May I take relief from a bridge, sprinkler control box or other immovable artificial object when my ball is in a penalty area?

A. No. When your ball is in a penalty area, you do not get free relief from an immovable artificial object (see Rule 16.1a(2))