Tarball and MercurialTo get one of these versions, go to the download page.
Under GNU/LinuxTo run this version, you have to install some packages. See README file.
Under Microsoft WindowsSee this page to install development version under windows.
Jabber/XMPPPlease read here.
StructureYou can understand how Gajim works here.
Gajim uses a fork of the xmpppy library to do the interface with Jabber network.