Joomla 2.5.18 is a maintenance release. Most notably this release fixes errors that have been reported including:
- Articles missing for admins in front end
- Modules disappear at frontpage after clicking the article title
Also with this release, the enhanced password hashing via PHPass, available in Joomla 3.2.1, is now used with the 2.5 series.
The update process is very simple, and complete instructions are available here. Note that there are now easier and better ways of updating than copying the files with FTP.