The Keepers guard the three scoring hoops on their end of the pitch.

Blocking the Quaffle
Blocking itself is a passive thing. The code automatically checks the scoring Chaser's stats against those of the Keeper. However, there are three possible outcomes of a score that actually goes near the hoops: It goes in, the Keeper blocks it without catching it (bats it away with some body part/their broomstick), or the Keeper actually catches the Quaffle.

+pass [name]
   Used to attempt a pass to another member of your team when you catch the Quaffle instead of knocking it away. Execute this command after your pose. If the Quaffle is freefalling (you have batted it away as opposed to catching it), it will automatically attempt a +get for you.
In Games: If a Chaser enters the scoring zone but does not attempt a score within three turns, the code will allow a Keeper to try and steal the Quaffle away. To do this, just use +pass as usual.
(Available in game and practice code)

+steal
   To be used in order to try and steal the Quaffle from an opposing Chaser. Use this command before your pose and include the results. (There is no need to specify a target; there is only one Quaffle so there is only one player to steal from.)
During practices, this command works the same way as it does for Chasers. Use it to steal from any other player carrying the Quaffle.
During games, Keepers are the only ones who can use +steal. You can only steal from a Chaser in your team's scoring zone, and you must wait three turns before attempting to steal. You cannot try and steal the Quaffle as soon as the Chaser enters. The three-turn rule is enforced by code (+steal is not available until a Chaser in possession of the Quaffle has been in the scoring zone for three turns).

NOTE: It IS possible for a team to manage more than one score attempt per round, if they get ahold of the Quaffle after an attempt with enough poses left to try for another one. The Keeper still only gets one command per round! Therefore, strategize carefully: If you only knock the Quaffle away or if it goes through the hoops, do you want to risk wasting your one command on trying to +get it when you might want to use +pass later?

If you are hit with a bludger:
Grazed: Miss one turn.
Hit hard: Miss two turns. If you are currently in possession of the Quaffle, you drop it. Three hard hits puts you out of the game.
Knocked off broom: You are out of the game.

If a Keeper is hit with a bludger, the code assumes that there is no Keeper while they're sitting out.

The code automatically assumes everyone is always doing their best to dodge bludgers, so there is no command required to avoid them. The message regarding whether or not you were hit will be emitted to you after your turn.

+skip turn
   Use this on your turn if you have been hit with the bludger (two rounds in a row if you are hit hard). The code will not permit you to execute any other commands until your number of missed turns are up.
(Full game only - NOT available during practice)

For your reference...
All three of these are only available to you during games.

+lineup
   During a full game, this command will bring up the player lineup for the two teams that are currently competing. It also lists their positions. If you have ANSI colours turned on, players who are actually connected will have purple names.

+qhelp
   For those players struggling to remember which commands are available to them during a full game, typing this command will bring up a short list of what they can use, along with a short description of the command.

+report
   During a full game, this will bring up a list of statistics for the game in progress. Basically, it gives a report of who has the Quaffle, where the Bludgers are on the field and who has yet to take a turn, amongst other things.