Maid of Honour/Chief Bride’s maid:

(A special friend or sister of the bride.)
Assists bride with invitations and other planning details.
Pays for own attire if possible.
Signs and witnesses the marriage certificate.
Helps bride in arranging veil and dress at ceremony and reception.

Best Man:

Pays for own attire.
Responsible for grooms transport to the church.
Takes care of bride's ring until ceremony.
Signs and witnesses the marriage certificate.
Holds for safekeeping, any tickets, keys, etc. for the couple.

Bridesmaids:

Make up the train during the bride’s entrance for the ceremony.
Responsible for confetti and other such activities.

Groomsmen:

Pay for own attire.
Escorts bridesmaids during the ceremony.
Attends the bachelor's eve party if there is one.

Flower Girls.

(Filled by a favourite niece/cousin between four and eight years.)
She follows the bridesmaids down the aisle, carrying a bouquet or basket of flowers.

Ring Bearer.

(Filled by a favourite nephew/cousin between four and eight years.)
The ring bearer carries the ring pillow with the wedding rings attached (these days fake rings are attached to the pillow, the real rings are entrusted to the Best Man).

Ushers:

There are usually at least two ushers (almost always women), who escort guests to their seats. Sometimes it is the bridesmaids who fill the roles of the ushers, but these roles can be filled by non-wedding party members usually work colleagues, schoolmates or close friends of the bride.
Pay for own wedding attire.
Form the wedding party that leads the couples’ grand entrance during the reception.
Distribute wedding programs.
Check church for any items left behind.