Just to expand on what John said.
The Focus Hold button pretty much does what it says - while pressed it holds (or locks) the focus - so you can use AF to focus to a particular distance, then press focus hold and until you release it the camera will not change the focus distance.
On some cameras you can actually use it as a custom button, and assign other functions to it - though the details on how may be somewhat hidden in the menus and manual.
A good place to look to find out more on specific lenses is the Dyxum lens databasehttp://www.dyxum.com/lenses/index.asp
This allows you to search for lenses by a range of factors, and will have information and user reviews on them.