Patch is a description of differences between the old and the new versions of the same file, introduced in a special format, which lets the patch utility to run the new version of the file based on the old version and the patch file. Normally, these are the text files with the .patch extension, and they look like this:

Magento patches are distributed in the form of files with .sh extension. They are different from usual patches because they have the installation scenario added. It implements atomicity of patch installation and deinstallation actions. This scenario also logs installation and deinstallation in the app/etc/applied.patches.list file.
In general, to install a Magento patch, you need to upload the patch file into the root directory of your website and run the following command. Don’t forget to replace with the name of the actual patch you’re going to install:
The different patches released so far for all the versions are downloadable from here