OK, so perhaps 'party' is too strong a word.
An event at which GPG users get together and authenticate each others identity to build the WebOfTrust. Usually a minor part of LUG meetings, conferences (particularly Debian and KDE conferences, because these groups rely on GPG authentication as part of their organisational infrastructure) or other gatherings of the cryptographically aware. Make sure that you up-load your key to the appropiate KeyServers before you get involved in KeySigningPartys, because people will expect to find your keys there.
If you really can't make it to a keysigning, the next best thing is to send your key to a RobotCA, which doesn't provide verification of who you are, but verifies that a holder of your PrivateKey collects email addressed to your email address.