The bowline is an ancient and simple knot used to form a fixed loop at the end of a rope. It has the virtues of being both easy to tie and untie; most notably, it is easy to untie after being subjected to a load. The bowline is sometimes referred as King of the knots because of its importance. It is one of the four basic maritime knots.
To attach a rope to many different objects.
To attach a rigging carabiner to rigging line when using webbing slings.
*Note: This knot isn't recommended to be used as the termination knot for a tree climber as it doesn't cinch onto the carabiner.
This knot is one of the most well known knots around the world as it can be heavily loaded and still easily untied and has other variations for different tasks.
Pictures and video courtesy of www.climbingarborist.com