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GNU Privacy Guard (GnuPG)

Basic GnuPG

Besides the most useful command shown below, there is of course the possibility to interactively edit your key-ring:s "gpg --edit-key $KEYID". Though if you enter "?" you'll get a list of possible commands, this list is not a complete one (e.g. it misses commands such as "expire", etc.). Check out man gpg for all possible commands you can use here.

But now, here's most of the stuff which should suffice in everyday life:

Generate a key-pair:
  gpg --gen-key

Upload the key:
  gpg --send-keys --keyserver pgp.mit.edu $KEYID


Revoke a key:
 Generate revocation certificate:
  gpg --gen-revoke $KEYID
 Save the certificate-block in $FILE 

 Import the revocation certificate:
  gpg --import $FILE
  
 Check the key is correctly revoked:
  gpg --list-sigs $KEYID
 Should say "[revoked]" in the first line and "rev" in the second
 Or:
  gpg --edit-key $KEYID
 Should say "trust: -/r"

 Upload the revoked key:
  gpg --send-keys --keyserver pgp.mit.edu $KEYID
  

List all keys:
  gpg --list-keys

List a specific key:
  gpg --list-keys $NAME
 or:
  gpg --list-keys $KEYID
  

Export the public key (as text-file):
  gpg --export --armor $KEYID > pubkey.txt

Export the private key (as text-file):
  gpg --export-secret-keys --armor $KEYID > pubkey.txt

Import a public or private key:
  gpg --import $FILE
  gpg --recv-keys --keyserver pgp.mit.edu $KEYID


Interactively edit a key:
  gpg --edit-key $KEYID


Encrypt symmetric (very strong):
  gpg --cipher-algo RIJNDAEL256 -o $CRYPT-FILE -c $PLAIN-FILE

Decrypt symmetric:
  gpg --cipher-algo RIJNDAEL256 -o $PLAIN-FILE --decrypt $CRYPT-FILE

GnuPG's ElGamal signing keys compromised

Though it is highly unlikely you have such a key: ElGamal sign+encrypt key (type 20) if used with GPG 1.0.2 or later must be considered compromised.
Here is Werner Koch's detailed explanation, including an easy way to double-check that you don't have such vulnerable keys.

Resources

An Overview of GPG.

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