Shear as it deals with hurricanes (and in the most general sense) describes the change in winds -- both direction and speed -- with height. You can also have horizontal shear, where the winds change in direction and speed over a horizontal area, but we are usually more concerned with vertical shear.

It is measured simply by taking wind observations throughout the depth of the troposphere (from the surface to ~10-15km up). Generally, the upper level and lower level winds are subtracted (using vector subtraction) to come with the shear magnitudes. You can also see mid-level shear calculated, where winds at the high and low end of the mid-levels (generally 300-700mb) are subtracted.

Shear values under 10 m/s or 20kt are necessary for any significant development....even lower is better. You will never see a storm with no wind shear; that describes what we call a barotropic environment, one believed to exist in theory only. But, you can see storms with very low (1-3kt) wind shear values; these are often the most intense storms and what we call "annular hurricanes." Mitch (1998) and Isabel (2003) are two good examples of such a storm.

Shear has a negative effect on a storm by disrupting the vertical structure of a tropical cyclone, affecting the convective pattern associated with the storm, and impacting the outflow at upper-levels requisite for the storm to survive and thrive.