The traditional receiving line allows the Bride and Groom and other members of the bridal party to officially meet and welcome all of the guests.  Although this personal welcome will be brief it is the only way to guarantee all guests are personally welcomed.
 
Once the ceremony has taken place, guests will make their way to the reception and it is here they are greeted by the wedding party, usually after a welcome drink and before taking their seats for dinner.
 
The traditional order is:
 
Mother of the Bride
Father of the Groom
Mother of the Groom
Father of the Bride
Bride
Groom
Chief Bridesmaid
Best Man
 
The advantage of the above order is that the Bride's Mother and Groom's Father are likely to know most of the guests.  If this is not the case the guests should introduce themselves to the first greeter and state their relationship to the now married couple, each greeter introduces the guest to the next greeter in line.   It is tradition to shake hands or kiss and say thank you for coming, whilst the role of the guests is to compliment those in the receiving line.  

Don't be afraid to change the receiving line layout, just as you would the Toptable, dependent on family circumstances and relationships.  It is also becoming more popular for the Bride and Groom to be the only greeters - this not only saves time but it is you the guests are there to see.