What's the difference?
Answer: About -3dB or half the signal level.
 

 

Circular and Linear polarization refer to the characteristics of the radio wave that is transmitted by the satellite towards your dish/LNB. Below are links to visual comparisons of Circular (Left or Right-hand) and Linear (Vertical or Horizontal) polarization.
http://www.barco.com/projection_sys...r&circulair.jpg
http://repairfaq.cis.upenn.edu/sam/icets/VHB6-6.gif

Circular polarization refers to a wave of radio signal rotating in a spiral. This spiral can be rotating in either the clockwise ("Right-hand) or counterclockwise ("Left-hand) direction. Think of an approaching airplane as the plane's propeller is moving towards you.

The following example satellites (visible in North America) are transmitting with a circular polarized signal:

EchoStar 3 @61.5°W http://www.lyngsat.com/echo3.html
Nimiq 2 @82.0°W http://www.lyngsat.com/nimiq2.html
Nimiq 1/3 @91.0°W http://www.lyngsat.com/nimiq1.html
EchoStar 6/8 @110.0°W http://www.lyngsat.com/110west.html
EchoStar 7 @119.0°W http://www.lyngsat.com/echo7.html
EchoStar 1/2 @148.0°W http://www.lyngsat.com/148west.html

How do I know they are circularly polarized?
If you click on any one of the links for the satellites above you will notice that under the first column labeled as "Freq. Tp" you will find one of the following single letter designations:

"R" = right-hand (=circular polarization)
"L" = left-hand (=circular polarization)

So what about linear polarization?
Linear polarization refers to a wave of radio signal rotating in a single plane. Think of an approaching helicopter as the helicopter's main rotor is moving towards you. It is rotating in a single, horizontal plane. The same approaching helicopter's smaller rear rotor is rotating in a single, vertical plane. In our satellite case it can be either in the "Horizontal or "Vertical plane.

The following example satellites (visible in North America) are transmitting with a linear polarized signal:

SBS 6 @74.0°W http://www.lyngsat.com/sbs6.html
AMC 5 @79.0°W http://www.lyngsat.com/amc5.html
AMC 9 @85.0°W http://www.lyngsat.com/amc9.html
IA 6 @93.0°W http://www.lyngsat.com/ia6.html
IA 5 @97.0°W http://www.lyngsat.com/ia5.html
AMC 1 @103.0°W http://www.lyngsat.com/amc1.html
AMC 2 at 105.0°W http://www.lyngsat.com/amc2.html
EchoStar 9 @121.0°W http://www.lyngsat.com/echo9ia13.html

Again, under the first column labeled as "Freq. Tp" you will find one of the following single letter designations:

"V" = vertical (=linear polarization)
"H" = horizontal (=linear polarization)
 

By using this link http://www.lyngsat.com/america.html you can determine for yourself what type of LNB you will need for each satellite.
 

If you need a circular polarized LNB look for words describing it as "DSS" or "DBS" or "circular" or a combination of these terms.

If you need a linear polarized LNB look for words describing it as "FSS" or "FTA" or "linear" or a combination of these terms (FYI: All Universal LNB's are linear LNB's).