How to use BJT as a switch?

During lecture on transistor modes, we discussed transistor works in different modes subject to change in current at the base. In some applications BJT works in one mode and in another application transistor works in another mode. For example transistor as an amplifier works in active mode.

When we want to use transistor as a switch we have to operate it between saturation and cutoff mode. We also discussed during lecture on BJT that base plays a pivotal role in the transistor working. If we change current on the base, we can switch transistor from one mode to another.