An excellent general purpose knot for tying two pieces of string or twine together, the reef knot is possibly the most commonly used knot for the job, and is easy to learn. However, it cannot be overly stressed that the Reef knot is not a long term or secure knot, and it should only be used to finish parcels or bindings. In other cases, use a more secure method of bending two ropes together, such as a Sheetbend, a Double Sheetbend, or a Fisherman's Knot.
Unfortunately, the Reef knot can easily change into a slipping Lark's Head (see below), so it should never be used where life or limb are at risk.
Holding one end of each rope in each hand, pass the left rope over the right, and tuck under. Then pass the same rope, now in the right hand, over the left rope, and tuck under.
It is common to chant "Left over Right and Under, Right over Left and Under" when tying the knot. (This can also be performed as "Right over Left and Under, Left over Right and Under".)
The reef knot can easily be undone by gripping one loose end, and pulling it back over the knot, in the opposite direction, thus straightening the rope which is pulled. The other rope forms a Lark's Head knot, and slips off the tugged rope.
The knot gets its name from its use on sailing ships, when the sails were "reefed" - rolled up and tied to the cross spar with a reef knot. To release the sail, the sailors would climb the rigging, and work their way along the cross spar, pulling the top end of the reef knot down. They only had to use one hand, holding on with the other. The weight of the sail would cause the reef knot to slip, and the sail would be released.

Tip. If you want to tie two ropes together of similar thickness then never use a Reef knot. Only use it with string and twine when tying parcels, whippings and bindings.
Tip. Never use this knot to join ropes of two different thicknesses.