This is standard practice for anyone that does restoration work.  What Fons has said, not to demean it at all, is common knowledge for those that use the tools.  Your standard 4 tools are a broadband de-noiser, used for relatively constant noises in the background, often a multiband expander at least when dealing with realtime applications.  A De-Buzzer designed to take at periodic noises that have relatively high harmonic content at regular intervals, primarily a series of carefully designed notch filters.  A De-Hum which is designed to take out periodic noise without much harmonic content.  And a De-Clicker designed to remove clicks and pops you get from vinyl and other sources (ie. Dust on Tape going over the heads sometimes).  But also as he mentioned you can get more specialized tools as well and yes some will go so far as to allow you to focus on specific spectral areas, if anyone has seen Audition this does this, but really I am referring to much more specialized tools that cost much more money, like Algorithmix Renovator.

There is an 'ok' article on the basic process located on CEDAR Audio's website, who are pretty much the standard in the industry to my knowledge for realtime noise reduction(As compared to Algorithmix reNovate which is offline/non-realtime)

http://www.cedaraudio.com/
Click on "About Audio Restoration"

At least it covers the basics pretty well if you click through to info on the various tools at the bottom of the page.

        Seablade