Active Devices are components that are capable of controlling voltages or currents and can create a switching action in the circuit. Examples are Diodes, Transistors and Integrated circuits. Most Active components are semiconductors. More on this in later sections. It has not control action and does not require any input other than a signal to perform its function.
Diodes are basically a one-way valve for electrical current.
If the negative side is on the negative end of the circuit, current will flow. If the negative is on the positive side of the circuit no current will flow. More on diodes in later sections. LEDs are simply diodes that emit light of one form or another. They are used as indicator devices.