an rail to rail opamp is one whichs output and input can swing almost to both the supply rails... actually it is can go to about 0.2V near the supply rails...
for an normal opamp it can generally go to one diode drop or 2 diode drop near the supply rails....
Input rail to rail means that it the input voltage can approch both supply voltage before going to an unknow state.
Output rail to rail means that the ouput is able provide a maximum voltage of VDD and VSS.