FourThirds are DSLRs, only small difference being the slightly smaller sensor. Means body can be a fraction smaller, and the effective reach of lenses is longer if you ignore DoF equivalence. While Olympus recently launched a new high end body, the format is for practical purposes dead and has no future.
MicroFourThirds is the mirrorless version, and the first mirrorless system around. Same sensor size, no mirror. The shorter distance between mount and sensor means for wide(r) angle lenses, the design is a bit easier so lenses can be smaller than SLR equivalent.
_________________Canon DSLRs: 7D, 5D2, 1D, 600D, 450D full spectrum, 300D IR mod
Lenses: EF 35/2, 50/1.8, 85/1.8, 135/2+SF, 28-80 V, 70-300L, 100-400L, TS-E 24/3.5L, MP-E 65, EF-S 15-85 IS
3rd party: Zeiss 2/50 makro, Samyang 8mm fisheye, Sigma 150 macro, 120-300 f/2.8 OS, Celestron 1325/13
Tinies: Sony HX9V.