A DMU or a diesel multiple unit is a diesel-powered train of multiple carriages (typically seen as a non-suburban or inter-city train). A DMU does not have a separate locomotive but uses (one or more) diesel-powered railcars (cars). Motorised cars ("M" cars) are usually semi-permanently coupled to non-motorised trailer cars ("T" cars) forming a "unit". Two or more units create a set.
e.g. an MTM unit consists of a traction car, a trailer car and another traction car, all semi-permanently coupled. an MTMMTM set consists of two MTM units coupled together for a given period of time
A single diesel-powered railcar may generally be classed as a DMU (even though there is only one car).