Why do batteries recover after a load is removed?
The actual process is dependent on the type of battery we are talking about.  In a lead acid battery, 


  The cell voltage will rise somewhat every time the discharge is
  stopped. This is due to the diffusion of the acid from the main body
  of electrolyte into the plates, resulting in an increased
  concentration in the plates. If the discharge has been continuous,
  especially if at a high rate, this rise in voltage will bring the cell
  up to its normal voltage very quickly on account of the more rapid
  diffusion of acid which will then take place.


from here.

In general, you can think of it as a normalizing of the chemicals involved.  There isn't any more "life" in the battery, the life remaining is just in the correct place to give you a little use.

Also, read this answer for basic battery info