If the cloud is relatively blue or blue/black, it may indicate that oil is being burned along with the fuel. The possible cause may be either worn piston rings or an oil viscosity that is too low.
If the cloud is black, it indicates that excess fuel is being burned.
If the cloud is white, it may simply be the moisture in the cold engine and exhaust system being burned off. If there's a lot of white smoke and it continues for a long time, you may have an internal coolant leak.