Basically an electrical filter is a circuit that can be designed to modify, reshape or reject all unwanted frequencies of an electrical signal and accept or pass only those signals wanted by the circuit’s designer. In other words they (Filters) filter-out unwanted signals, and an ideal filter will separate and pass sinusoidal input signal based upon their frequency.

Filters are usually classified according to filtering range, frequency response in pass band, and circuit component. Classified by filtering range there are four types of filter:
   1) Low Pass Filter
   2) High pass filter
   3) Band Pass filter
   4) Band Reject Filter
According to frequency response in pass band there are two types :
   1) Active Filters
   2) Passive Filters

Active Filter :- An active filter uses amplifier along with resistors and capacitors to do the filtering while passive filter do not contain any amplifying devices to strengthen the signal.
Passive filter :- As there are two passive components within a passive design the output signal has a smaller amplitude than its corresponding input signal, therefore passive RC filters attenuate the signal and have a gain of less than one (unity). Passive filters are made up of passive components such as resistors, capacitor and inductors and have no amplifying elements (Transistor, op-amp etc), so have no signal gain, and therefore their output level is always less than input.
As the function of any filter is to allow signals of a given band of frequencies to pass unaltered while attenuating or working all others that are not unwanted.