several coefficient differences in UV-vis Spectroscopy
In most applications, these terms are used interchangeably.

Most absorbance spectrophotometers work by measuring the intensity of the light beam that is transmitted through the sample.  A strong light beam goes in, and a weaker one comes out the other side.  The transmission is the fraction of the light that made it through.  

There are two main ways in which the light that goes in gets diminished in intensity before it goes out.  The first is absorption.  Molecules that absorb light of a certain frequency make photons of that frequency disappear.  The energy of those photons excite electrons into higher energy states.  These states may eventually decay, releasing heat or lower-energy photons, but not photons of the same energy that were absorbed.  The propensity of molecules to absorb can be called the absorption coefficient.

Scattering just deflects the incident light to a new angle.  Now, when it exits the sample, it won't come straight out on the side opposite where it went in, and it won't be detected by the detector.  Molecules can cause scattering if they are very large.  The "scattering coefficient" describes a molecule's propensity to scatter light of a certain frequency.

The "extinction coefficient" is -- in most applications -- just equal to the scattering coefficient plus the absorption coefficient, i.e., it's proportional to a molecule's ability to disrupt incident light in some way or other, regardless of the mechanism.  

In the usual, transmission-based spectrophotometer, only extinction coefficients can truly be measured.  To resolve these measurements into scattering contributions and absorption contributions would require measuring the intensity of the scattered light, i.e. detectors would need to be present at multiple angles.