The goalkeeper (also called the 'keeper or GK) adheres to the same rules (Laws of the Game, or LotG, or just Laws) as any outfield player, with a few exceptions (all of which are also found in the IFAB Laws of the Game, published periodically by FIFA). Some competitions (particularly in the cases of leagues or divisions for younger, older, disabled, or women players) modify the rules.
Note: The masculine neuter pronoun is used here for simplicity, but applies equally to men and women.
1. Uniform / Kit: The GK must wear a jersey that is different in color than his teammates, his opponents, the other 'keeper, and the match officials. He is permitted to wear track suit bottoms (instead of shorts, as required for other players). He may wear other protective equipment, such as padded gloves or knee and elbow pads, subject to approval of the league or competition, the referee, and any safety concerns for himself and for other players.
2. Handling: The goalkeeper is allowed to handle the ball under certain conditions; he is the only player allowed to do so. The goalkeeper is allowed to touch or hold the ball with his hands and arms if and only if: both he and the ball are within his own penalty area (the 18 yard box surrounding his team's goal), the ball was not kicked to him by a member of his team (including himself), and the ball was not directly received from a throw-in.
Once the goalkeeper handles the ball and gains possession, the opposing team must give the goalkeeper time and space to release the ball back into play; the GK cannot be hounded, chased, tackled, or otherwise challenged while holding the ball. The GK must get rid of the ball within six seconds, though in practice this time limit doesn't start until the GK is able to actually rid himself of the ball; this means that the GK gets a little extra time to stand up after a dive, and doesn't get penalized if an opponent is nearby and causing interference.
3. Penalty Kicks: The goalkeeper must wait with both feet on the goal line and between the goal posts until the kicker kicks the ball. This makes his job exceedingly difficult, which is why penalty kicks are so valuable and often successful. The same applies to "Kicks from the Penalty Mark", a tiebreaker sometimes used in matches where there must be a winner. The other goalkeeper (the one on the current kicker's team) does not stand with his teammates in the center circle.
4. Substitutions: When a team wants to substitute its goalkeeper, or if the 'keeper and an outfield player want to switch roles, extra time can be allowed for the uniform/kit change. Outfield players may change positions and responsibilities among themselves at any time, but on-field 'keeper changes require referee permission and must occur at a natural stoppage in play (except in case of injury, in which case the stoppage will be for that injury).
5. Readiness: In most competitions and leagues, the referee will wait to start play until both goalkeepers are ready. There have been international matches where a free kick was postponed so that a 'keeper could retie his shoes. The referee is also more apt to stop play for an injured GK (assuming no foul was committed) than for outfield players, but this comes from the fact that the Laws state that only the referee may determine whether to stop play or allow medical treatment for any injury.
Other than the above, the goalkeeper is exactly like any other player on the field. Some people believe that the 'keeper can get away with more brutality or that a foul against a 'keeper is more likely to get called or get a more severe punishment, but this is only partly true. It is the nature of the game that some of the fiercest and most passionate gameplay occurs within 15 yards of the goal - the goalkeeper's domain - which means that the 'keeper is statistically more likely to be involved in the intense and dangerous plays. Additionally, the GK routinely performs some of the most athletic and dangerous feats in the game, putting body and limb on the line by jumping or diving to make a play in the middle of a pack of opponents kicking their leg at maximum force. However, the Laws of the Game do not provide for any exceptions to force and brutality rules and therefore they do not exist (some leagues or competitions might make exceptions, but the world at large does not).