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The Surface Acoustic Wave filter is an electro mechanical filter. It consists of a crystal with electrodes connected to it.

-----.           .---- 
    _|_         _|_
    "T"         "T"
-----'  crystal  `----

When a signal of a 'pass through' frequency is connected to the input, it causes the crystal to expand or contract (but offcause only a microscopic change. This effect is called piezoelectrics). The wave created by this travles trough the crystal and create an electric signal on the output. By altering the shape of the crystal and trimming it down with a laser cuter, a highly complex filter can be created.

The SAW filter is a small solution to a complex filter device, but it attenuates the signal with about 30 dB at it's peak pass-trough. This means the output signal must be amplified before use.

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