Discharging a lead-acid battery converts the sulfuric acid and metallic lead inside into lead sulfate. Charging it changes the lead sulfate back into metallic lead and sulfuric acid. Leaving a battery discharged for a long time promotes the formation of lead sulfate crystals on the lead plates. When the sulfate crystals get large enough, they'll form an electric short between the plates resulting in a dead battery.

Bottom line is as Mr. Ramsell said: charge it right away after using it!