This section deals with the requirements for obtaining a commercial driver's license and keeping it.  It's definitely the condensed version of the regulations, so if you want to read the real thing and get much  more information, go to the website and read to your heart's content.
http://www.fmcsa.dot.gov/rulesregs/fmcsrhome.htm

Before you can obtain a CDL, you must meet the following requirements:
     1.  Qualify under Part 391 of the FMCSA regulations.
     2.  Pass a written knowledge test about the type of vehicle you expect to be operating.
     3.  Pass a skills or driving test in a vehicle that is the type you expect to be operating.
     4.  Surrender your non-CDL driver's license to the state. 

If you operate a commercial vehicle, you may not at any time have more than one driver's license.

You may not operate a commercial vehicle unless you have passed written and driving tests set forth by the standards of the FMCSA.

If the holder of a CDL receives is convicted of a traffic violation, other than parking tickets, in a state other than where he/she is licensed; he/she must report that conviction to his/her licensing state and his/her employer within 30 days.

If the holder of a CDL has his/her license suspended, revoked, cancelled or has lost any driving privileges, he/she must notify his/her employer immediately (for obvious reasons.)

When applying for a driving position, employers are required by statute to obtain certain information about the applicants employment and driving history, and the driver is required by statute to provide that information (if he/she wants a job, that is.)
     1.  Employment history for the 10 years preceding the application for employment.
     2.  List of names and addresses of applicant's previous employers for which the applicant operated a commercial vehicle.
     3.  Dates of employment with previous employers and reasons for leaving.
     4.  The applicant must certify to the truth of the information.
     5.  The employer may ask for further information and contact previous employers for verification of information and for information on applicant's work history.

Drivers may be disqualified from driving for the following reasons:
     1.  Driving under the influence of alcohol or a controlled substance.
     2.  Refusal to undergo required testing for alcohol or controlled substances.
     3.  Leaving the scene of an accident involving a commercial vehicle.
     4.  A felony involving the use of a commercial vehicle.

Drivers convicted of any of the above offenses will be disqualified from driving for:
     1.  First offenders: 1 year (as long as the vehicle was not transporting hazardous material.)
Subsequent offenses:  Life.
     2.  First offenders (HazMat):  3 years.
Subsequent offenses:  Life.
     3.  First offenders (Controlled Substance Felonies):  Life.

U.S. Department of Transportation
Federal Highway Administration