Patch Tuesday refers to the second Tuesday of every month when Microsoft releases security patches. The patches are pushed out by Microsoft's Windows Update (and Microsoft Update) services. Security patches are accumulated over the preceeding month (unless they are particularly critical or unless there are working exploits in the wild for known vulnerabilities) while IT administrators then have time to prep for updates to their networks of Windows machines.
Most security patches that are distributed on Patch Tuesday every month are generally unknown outside of Microsoft and the reporting security firm or expert until after the patch has been released. However, if a working exploit for a vulnerability shows up online and there is evidence that it is causing significant problems, then Microsoft will go out of the Patch Tuesday cycle and release an update.
Following Patch Tuesday there are often an increasing number of attacks against unpatched systems as the secrets of the vulnerabilities can be found out. This is often called Exploit Wednesday.