Goalkeeper is one of most important players of a team. Goalkeeper has a very big influence in the defence ratings.
Primary skill they use is Goalkeeping (or keeper) and secondary skill is defending, although Set Pieces is also important, but only to save free kicks and penalties.
Regarding influence on ratings, a level of keeper is equivalent to about 3 levels of defending for a goalkeeper.
Some teams used to choose their goalkeeper as a Set Pieces taker. There was no penalty for doing this, but it was disallowed as an option in season 37 (October 2008).
George 'Archbishop' Carey is probably the greatest goalkeeper ever, having achieved a 22 stars record.