Usually a patch is an update or bug-fix.
Patch is generally much smaller than full version.
For example you may see that there are listed Multimedia Xplorer version 2.0.4 and also Multimedia Xplorer 2.0.4 patch . The difference is that full installation contains full version, but the patch contains only changes between those 2 releases. If version 2.0.4 was released, then the program contained some modifications or fixes compared to version 2.0.3. The only way users of version 2.0.3 can get new, updated program is to download it again. Instead, they only need to download the small patch, that applies the changes to the version they already have - patch converts their version 2.0.3 into version 2.0.4.