theres lots of software that you can use to do it
basically (very very basically) its like this:
you take a picture with low exposure
you take another picture of the same thing in the exact same location with medium exposure
and you take another one with high exposure
(most people who do this though take closer to 10 different exposure settings)
then you blend the 3 together to make a picture that shows in clear detail every single part...it eliminates shadow spots that you cant see into and really bright white spots that you cant see into from over exposure...and it gives some really cool pictures as a result
take this picture for example:
http://en.wikipedia.org/wiki/Image:New_York_City_at_night_HDR.jpg
notice that you can see in dark areas and light areas? everything has crisp and clear details...that is the result of HDR processing
heres the best example I could find, sourced from this site:
http://www.cambridgeincolour.com/tutorials/high-dynamic-range.htm
(that sites got a lot more information about it as well if you are more interested)
under exposed shot:
over exposed shot:
the result of combining the two:
what I think happened above to mhrebin is between the different shots the camera got bumped ever so slightly, which caused that 'lumpyness' and strange coloration differences