The way I would look at it is this,
You are in cloud if:
a) it looks like a cloud and you're in it,
b) the forecast/report actually says there are clouds at your level such as ST, SC, CU, etc and your path takes you there,
c) FG is fog which is cloud that touches the ground, so technically you would be flying in ST cloud if you were flying in FG, (and before anyone jumps up and down, I am not differentiating between radiation fog, advection fog, etc... I'm just trying to make a point about cloud),
d) FU is smoke haze which is not cloud in the sense we know it but which can reduce visibility to below VMC,
e) RASH is rain showers which is not cloud (unless you are up in the ST or CU or whatever) but the RASH can reduce visibility to below VMC.
Anyway, the point is VMC means visual and flying in cloud or other obscurations usually means you are not flying in accordance with the rules of VFR.
I'm sure there are plenty of other people who would like to add some more input. The floor is yours...