. passive filters, made of passive components: resistors, capacitors, and inductors; 2. active filters, employing Operational Amplifiers.

There are 2 main types of filters:
1. passive filters, made of passive components: resistors, capacitors, and inductors;
2. active filters, employing Operational Amplifiers.

Passive filters are built with:
A. Resistor-Capacitor: these are RC filters, and they are the most used ones since they are easier and cheaper to build;B. Inductor-Capacitor: they are noted as LC filters, and they have better performances. The problems are: inductors are expensive, very difficult to tune to exact values, and they may require shielding of their electromagnetic field.

Both the passive and the active filters may be serialized (cascaded): in this way we obtain 1, 2, 3 .. n order filters. Of course, the higher the order, the better is the filtering. Commonly, we build a 3 up to 9 order analog filter, and then we use a digital firmware filter (a DSP filter) of 500, 1000 or even 2000 order. Again, although digital filters--they are in fact firmware and software routines--are way more efficient, they do not work very well if you do not have a first, basic, analog hardware filter.

Depending on their functionality, both passive and active filters can be:
1. Low-Pass
2. High-Pass
3. Band-Pass
4. Band-Reject