You rack into it, just like you would rack into a bottling bucket. The is a large lid on the corny kegs that comes off so that you can fill/clean them.
Greg, most homebrewers use old five gallon soda kegs, called cornelious (corny) kegs, to keg homebrew:
You CAN keg in a normal commerical beer keg, called a sankey keg, but it is a PITA to take the spear out of the keg and replace it. Not hard to do, just a pain compared to a corny keg, where the top just pops right off.