A linear polarizer is a device which selectively allows the passage of only certain orientations of plane polarized light. At one orientation it might allow the passage of only vertically polarized light, while if rotated by 90 degrees it would allow the passage of only horizontally polarized light. Half way in between, at say 45 degrees rotation, it would allow passage of only 45 degree plane polarized light. 

This Linear polarizer is used to saturate colours, darken blue skies & reduce reflections. A Linear polarizer will have a stronger effect to a Circular polarizer as a rule.

Linear Polarizers can affect the exposure and in some cases the auto focus function of your camera. Check your manual to see if a Linear polarizer can be used.

"However, the linear design can interfere with metering &/or AF in newer generation SLR & DSLR’s. (When testing for this with a linear PL on a Kodak Pro SLRn I didn’t encounter a situation where the AF failed to work properly, but exposures tended to be a bit light)."

This filter is supplied in a Twist box for compact and safe storage