The goalkeeper is subject to the Laws of the Game as other players are when he is outside of his own penalty area. Inside his own penalty area, he may play the ball with his hands, but there are instances when he cannot (even though he is within his own penalty area). Let's look at those.
If for some reason one of his team-mates throws the ball in to the keeper and the ball does not touch an opponent before coming into the penalty area, the keeper may not use his hands to play it. This is most unusual, but it is covered in the Laws.
Additionally, if a team-mate intentionally passes a ball back to his own goalkeeper by kicking it, the keeper cannot play it with his hands even though he is within his own penalty area. (He may play a ball with his hands that is headed to him, however.) This portion of the Laws can be seen in action frequently in a game.
If a keeper puts a ball back into play by dropping it or rolling it on the ground in front of him, he cannot again pick it up with his hands even though he is within his own penalty area. (He has put the ball in play; he has put in on the ground to kick it and must now kick it.) This isn't something that happens often, but it can with an inexperienced keeper.
If a keeper is playing a ball outside his own penalty area (and he'll be playing it without using his hands, of course), he may maneuver the ball back into his own penalty area and pick it up. Assuming he did not receive it directly from a kick or throw-in by a teammate.
Oh, and you already know this one instinctively. The keeper must dress in a way as to be easily differentiated from his team-mates, his opponents and from the officials. And for exactly the reason you think.
Now you're up to speed. In case something else comes up, the link provided will take a surfer to the FIFA Laws of the Game. And if it isn't clear, there's always Wikianswers for a backup. See you on the field. I'll be the one wearing black and carrying a whistle.
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