Data Comm is getting out of the modem pool business - faster access can be provided in other ways (Cable Modems, DSL) - but here are some links that may be useful for those of you using and considering modems.
What you need to know about modems provides more general detail and terminology.
The modem dictionary defines many commonly used modem terms.
The Hayes technical reference manual tells you more than you ever wanted to know about the Hayes AT command set.