A diode refers to any electronic device that allows current in one direction only. When a diode is used, a very negligible amount of current is lost, although this is only when low current is passing through it. However, if the supply is high, it will equally reflect in the amount of current being lost. It works by allowing current to flow only if it is forward biased and also blocks current from flowing if it is reverse biased.
On the other hand, a rectifier is also functioning as a diode just that it can only be used to convert alternating current into direct current. Due to this, a rectifier is sometimes called an inverted. Before a rectifier is used, it will be placed on any of the two lines of alternating current. While a rectifier is used mostly when converting from AC to DC, a diode, in terms of application, can also be used as a switch.