Mobile CB antennas are either a magnet mount, mirror mount, trunk mount, or a direct screw in or bolt style.  CB mobile antennas need to be grounded to the vehicles body, because at 27mhz, a 1/4 wave electrical length is just under 9 feet.  Antennas need to be 1/2 wavelength or larger to be tuned correctly, so the metals of the vehicle acts as the other half of the antenna, via being a ground plane.  This is the reason, when often placing a mobile antenna from one car onto another, it must also be retuned for proper standing wave ratio. 

The Wilson 5000 and Firestik antennas are the most popular.  The Wilson 5000 is expensive, however is one of the top performing antennas made today.  A more affordable version is the lil Wil, the Wilson 1000 magnet mount, and the Wilson 2000 trucker series.  The K40 is another favorite, and have some new models for truckers recently, to compete more with Wilson.  There is also competition class antennas known as Monkey Made, but difficult to find in most truck stops. 

Truckers may use the "dual" setup versus a single antennas, as the trailers metal blocks radio frequency signals from radiating trough them.  This is often why, a trucker driver can often talk further to those in front of him, but poorly behind him.  The dual setup helps widen the signal, and has a larger probability of counter effecting this shortcoming.