The clove hitch is a good knot to use to tie a line to a pipe or post.

How to tie a clove hitch
Take the running end of the line and wrap it around the post twice, the first time the running end should end up below the standing part, the second time the running end crosses over to be above the standing part. Finish the knot by slipping the running end under the crossover and pull to tighten.

You can also form a clove hitch in the middle of the line by making to adjacent underhand loops and slipping them over the post.

The clove hitch is a great knot to know, no doubt. However, they're dangerous if the line to whatever you're tying up alternates between slack and tension, because clove hitches easily fall open.

In other words, if you're hanging a brace of rabbits from a tree branch, a clove hitch works great. Don't use one to moor your boat, though.

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Gorgonzola suggests that if you follow a clove hitch with a couple of half hitches, your boat is safe. I still think a bowline is simpler.

A clove hitch can also be looked at as two half-hitches tied around the post, instead of the attached end of the same rope. Clove hitches are often used to start a lashing.

An ASCII art rendition:

                #########
                #########
                #########
                #########
               ______  ___
              /      \/   \
              \____  /   _/
__________________ \/   /
                  \/   /\______
________________  /   /        |
                \/   /\________|
               _/   /\____
              /    /      \
              \___/\______/

                #########
                #########
                #########
                #########

A good hitch for tying a boat to a pier might be the mariners' hitch.

HOWTO: Tie a Clove Hitch

A clove hitch is used to tie a rope to a solid object, such as a bar or branch, such that the rope will not slip when tension is applied to it. It consists of two half hitches made in opposite directions.

1. Place the rope across the bar from left to right.

      |    |
      |    |
  ____|____|____
A ______________ B
      |    |
      |    |
      |    |

2. Pass the right end (B) under the bar to the left side.

      |    |
  ____|    |
B ____|____|\
A ___________|
      |    |
      |    |
      |    |

3. Bring end B back to the right side and cross the rope on top of the bar.

      |    |
     _|_   |
  __|__ \__|\
A _____\ \___|
      | \ \|___
      |  \_____ B
      |    |

4. Again, pass end B under the bar to the left side.

      |    |
     _|_   |
  __|__ \__|\
A _____\ \___|
B ____| \ \|\
      |  \___|
      |    |

5. Finally, bring end B back to the right but pass under the crossover made in step 3.

      |    |
     _|_   |
  __|__ \__|\
A _____\ \___|___
    |___\ \______ B
      |  \___|
      |    |

A clove hitch can be tied very quickly with one hand. It is also easy to untie, even after a load has been applied to the rope.