D-registers and M-registers

Hello, this may seem like a really stupid question, but can anyone tell me exactly what the difference between D-registers and M-registers are? Or does it just depend on how the particular software uses them? Thanks

Are we talking Mitsubishi here or another PLC ?

Mitsubishi

D are data registers. They cannot be addressed on bit level in the FX series (except FX3U).
M are bits (also called auxiliary relays in the manual).

Just to expand that, the D registers are 16 bit registers only availlable as words, the M are bit level m0, m1,m2 etc.

Note in mitsubishi you can alter the settings to give the range of retentitive i.e. D200 to D512 or D0 to D512, you can do this for most addressable internals.
You can copy D words to M bits by mov instruction & vice versa i.e. D200 k4m0
the K4M0 means 4 nibbles (16bits) starting at M0.

Cheers thanks for your help guys