They're frequently the set pieces of the day ( Best man and bridesmaids, Brides family, Grooms family, Confetti shot, Cutting the marriage cake etc ). These, if you like, are the historic record. For some couples this could all that might be needed. Get some more information about table linen for wedding. These are miles more relaxed shots, catching the bride-to-be and groom and their guests during the daytime.
The shots attempt to build a tale of the day, and to get photos of the marriage couple and their guests as themselves and enjoying the day. The marriage couple may need to hire a paparazzo who can also add an inventive, up to date style to some or the lion's share of the footage. This will include fashion mag type shots, or changes to colour and contrast. How much coverage are you wanting? Are you wanting your shutter-bug to be with you from the instant you rise till the last dance or do you only need them at the celebration or till the 1st dance? Most photographers will have a variety of packages to fit your wants from rite only to full day coverage ( and some will even go past that if needed and join you on the stag night or include some studio or on location engagement shots in the package ). The web gives you a good chance to view the work of a good range of photographers and help you to build a shortlist of the photographers you wish to go to see. This will avoid dis-satisfaction from misunderstandings later on.