A classic knot, the bowline is traditionally used by sailors to affix a square sail to the bow (front) of the boat. With uses far beyond sailing, it is a loop that will not slip, and can be tied around objects or lassoed over them.
Knot Video: How to Tie the Bowline Knot
Knot Tying Instructions
Step 1 - Make a small loop, with plenty of length remaining to the right of it (a little over double the length you want the final loop to be).
Step 2 - Bring the end of the rope up through the loop.
Step 3 - Bring the end down underneath the main line, to the left of the initial loop.
Step 4 - Reach through the bottom of the initial loop, and pull the end through tight. The loop you've created below the knot is the useful part. It will not slip, and can be reliably lassoed or tied around objects.