What @Rangerr has listed out is perfectly right. I will help you out with the comparison between the standard options and the optional ones.
Pair 1: STD vs STD (O) - 1 Driver's airbag in STD and 2 Airbags, one for driver and one for the passenger.
Pair 2: LXI vs LXI (O) - 1 Driver's airbag in STD and 2 Airbags, one for driver and one for the passenger.
Pair 3: VXI vs VXI (O) - All features common
Pair 4: VXI AGS vs VXI (O) AGS - Power Windows only in the front window for VXI AGS but power windows in the front as well as back in VXI (O) AGS.
It says optional because one can choose some features which are optional.