Best Man

A Best Man is the groom's chief attendant at a wedding.

[edit] Variations

Some grooms choose not to designate a groomsman as a Best Man, or to have any groomsmen at all.
Some grooms choose a woman to be their Best Man, sometimes referred to as Best Woman.

Tips For Selecting Your Best Man

The first thing you should know when making your decision on who should be your best man is to not feel bad about your final decision. Not all of your male friends/cousins/siblings can be your best man, so don't feel bad when you have to choose.
Write a list of who are the top contenders. Near their name, write qualities that you feel your best man should have.
Talk to someone else about your choices, especially someone who you feel can make good decisions.
Choose the most responsible one since the best man will have his own responsibilities to attend to. (Look in the Responsibilities section of the article and see who can do these jobs well.)
If you really don't know, just let your friends decide as to who should be the best man.
If you still can't decide, you may even consider having best men.

The duties of the best man are as follows:

Give the groom assistance for whatever he needs on the day of the wedding.
Hold the rings before the ceremony and during the ceremony when the bride and groom are giving each other the wedding rings.
Act as a legal witness to the marriage.
Make a toast to the newlyweds at their wedding reception.
Organize the bachelor party.
Some are also responsible for getting certain things such as the suits, ties, etc. for the rest of the groom's wedding party.
Act as a go-between for the groom and give the bride her wedding gift from the groom on the day of the wedding.
Provide transportation for the bride and groom.