A bend is used to join two lengths of rope together by their ends.
Most bends work best when the ropes are of equal thickness else the knot may not be secure (one rope may slip out of the other). There are a few bends that can be used for joining ropes of unequal thickness.
Having joined two ropes together the bend is often placed under strain. Be careful to leave a long enough 'end' on the bend else when you put strain on the rope the short end may slip in and unravel the bend and the ropes will fly apart.
It is not just how easy they are to tie either. It is important that you can untie the knot easily, even if you have put a great deal of strain on the rope.
Ashley's Bend - Very secure, easy to untie, useful for very thin rope
Blood Knot - Useful for joining nylon lines. Can be difficult to untie
Carrick Bend - Excellent knot, used for all rope and cable
Fisherman's Knot - Simple, effective
Hunter's Bend - Useful for slippery rope
Sheet Bend - Simple, quick and easy
Water Knot - Strong and reliable. Useful for joining flat tape.