There are two kinds of assignment, continuous and procedural. Continuous assignments can only be made to nets, or a concatenation of nets. The operands can be of any data type. If one of the operands on the right hand side (RHS) of the assignment change, as the name suggests, the net on the left hand side (LHS) of the assignment is updated. In this way values are said to be driven into nets. Continuous assignments can be used to replace gate level descriptions with a higher level of abstraction.
Procedural assignments are made to reg, integer, real or time, they need updating constantly to reflect any change in the value of the operands on the RHS.