Bowline

One of the most versatile knots everyone must know.
The bowline is one of the most commonly used knots on the water and is used to make a strong loop in the end of a line.  The main advantage of a bowline over other knots is that it is easy to get undone again even after heavy loading or wet.  It does considerably weaken the line though - up to 60% - so should be used when ease of untying is more important than absolute strength.
 

STEP 1. Make an overhand loop in your line.  Make sure you allow enough tail to go around whatever you want the loop to go over.

STEP 2. Thread the end up through the loop (the rabbit comes out of its hole).

STEP 3. Thread the end around the standing line (the rabbit goes around the tree) and back through the loop (the rabbit goes back into its hole).

STEP 4. Pull the knot tight.  The bowline is known to work itself loose if not kept under constant tension so keep it tight if you can and make sure you give it plenty of tail (much more than in the picture)