A buffet table is usually used in the dining room and is used to lay food on in a 'buffet style." Guests then serve themselves from the buffet table, which can also bea cold table, salad bar or smorgasbord.
A sideboard can be used in any room. For example; it might be used to place a TV on top, to display family photographs or to display ornaments. A sideboard, as in its name implies is designed to sit at the side of the room.
There are not that many differences between a buffet table and a sideboard, it is more to do with the function it has been chosen to fulfil within the household. Both the buffet table and the sideboard may have the capacity to store table linen, extra cutlery or crockery.