BANDPASS FILTERS

Bandpass filters are designed to do exactly what the name implies, pass a narrow range of frequencies and reject those above and below the design frequency. There are different ways to design a bandpass filter which can affect the bandwidth (generally defined at the -3 dB power points), insertion loss at the desired frequency, and of course the attenuation level of signals falling outside of the passband (see how the words gets turned around to help explain it better).

You might be building a microwave transverter and need to attenuate the fundamental frequency of a crystal oscillator as well as some harmonics while allowing the desired harmonic through. This is easy to do with a bandpass filter. The design would be centered on the desired frequency, which would pass through with minimum attenuation, while substantially reducing (attenuating) the signal level of the undesired frequencies.

The ARRL Handbook is a great source of information on bandpass filters. You can also refer to many other ARRL and RSGB publications for additional information.