Filters are a group of very useful circuits that allow us to change the frequency content of a signal. They achieve this by allowing only some frequencies to pass, while blocking those that remain. This has a diverse range of uses, including power supplies, radio communications and audio circuits. If you imagine a filter as a block, the original signal goes in and the filtered signal comes out.
There are four main groups of filters: low-pass, high-pass, band-stop and band-pass. These filters can then be divided into further groups, passive and active. Passive filters use passive components (resistors, capacitors and inductors. Active filters use active components (transistors, op-amps).