The bride's mother traditionally had a more demanding role to play in the organisation of the wedding.  However in these days of financially independent young women (and many couples already living together), it is more often a supportive role to the couple with assistance in moderating the guest list, advising family and ethnic traditions, being a warm listener and chief cheerleader. 

The groom's mum can assume any of the bride's mum's responsibilities and assist the mother of the bride.

Best Man

This honoured role (like the maid of honour) is usually the groom's best friend or brother.  A trend of the groom's father taking this role is growing in popularity.  The best man is the groom's personal aide and advisor and generally responsible for things like making sure the groomsmen have their boutonnieres, paying the officiant after the ceremony, holding the wedding rings and toasting the bride and groom at the reception.

Groomsmen

Like the bridesmaids, groomsmen are men closest to the groom, being brothers,  best friends, cousins etc.   Their role is to help the best man plan and pay for the bachelor party, assist the groom get ready on the big day, escort guests and bridesmaids, decorate the getaway car and return formal wear to the hire company.

Father of the Bride / Father of the Groom

The father of the bride has the very important role of supporting his daughter on her wedding day.  He will ensure that she gets to the church on time, walk her down the aisle and give her away to the groom.  He will also have the honour of making the first speech after the meal at the wedding reception. 

The role of father of the groom is more general that that of the father of the bride, yet still important.  He generally will assist and mingle with guests, advise any specific cultural or family customs, offer support to the father of the bride and make a speech.

Flower Girls / Page Boys / Ring Bearers

Flower girls, page boys and ring bearers are a joy to have on your wedding day.   They are usually nieces and nephews or young brothers and sisters and generally no younger than about 5 and not older than 9 or 10.  It is generally recommended that children are not younger than 5 as they are unlikely to understand what is expected of them and may even decide to start playing hide and seek as you are exchanging your wedding vows.
Flower girls walk in front of the bride carrying posies of flowers or bunches of thornless roses.  They can also scatter rose or other flower petals before the bride as she walks down the aisle from a basket.
Pageboys traditionally carry the brides train if she is wearing one.  The ring bearer (or pageboy)  walks down the isle just before the flower girl and carries the wedding rings on a small decorative pillow to the couple.

Ushers

Ushers are generally there to ensure guests  are seated correctly at the ceremony.  Sometimes groomsmen fill in for this role.