Orthogonal time frequency space (OTFS) is a recently proposed modulation scheme which multiplexes information symbols in the delay-Doppler domain to combat severe Doppler shifts in high mobility wireless communications. In this paper, we classify all the OTFS variants depending on whether a cyclic prefix (CP) or zero padding (ZP) is added to each block or to the entire frame. We then present the general input-output relations for integer and fractional delays and Doppler shifts. Further, we present a low-complexity universal maximum ratio combining (MRC) detector for all OTFS variants, which has the lowest complexity among other known OTFS detection schemes at no loss of performance. Finally, we derive the input-output relation for overspread channels, where the channel delay spreads can exceed the OTFS block duration. Simulation results demonstrate that the MRC detection offers same performance for all OTFS variants and can also be effectively used for overspread channels.