Renaming one file is easy. Renaming a dozen is tedious. Renaming a thousand...get someone else to do it, or get ReNamer, a wonderful free/donationware program which allows you to set up fairly robust rules for renaming files in a batch.
ReNamer has a simple, straightforward, interface. You set up some rules--such as "Replace the file extension" or "Delete the last 4 characters"--and then select files to which to apply the rules. A very useful preview option allows you to see what you're going to get before you execute the process, saving unending amounts of grief and encouraging experimentation. Once you're satisfied with the results, ReNamer goes to work, renaming all the input files according to your rules.
ReNamer passes my most important test: Can you get it to do something useful without reading the manual? While there is detailed documentation (and I do not discount the utility of it), ReNamer is simple enough (if you are used to thinking in terms like prefixes, suffixes, and regular expressions) that you can do many common tasks without cracking the books...er...opening the PDF.
It's free. It's useful. It does one job but covers most variations of that job. I can see no reason not to add ReNamer to your toolbox.
Note: This program is donationware. It is free to try, but the author accepts and encourages donations towards further development.