What is Bandstop filter ?

The Bandstop filter is notch filter .. Tt's inversion Band-pass filter . But I don't know what's application.

The bandstop can also have a broad stop bandwidth. They are used to eliminate desired frequency ranges. One example would be to improve the IMD on your short wave radio by stopping the broadcast band signals.

It can surpress the second resonts for circuits!

the design of bandstop filter is similar with bandpass filter.

what's product will use this structure ?

You can download a free LC filter program from http://www.aade.com/filter.htm and then do some sample designs and graph the results. This is an entertaining way to learn what such filters can do.

For instance, try widening the stop band or increasing the number of poles and see how well it works.

Sometimes, you use bandstop (notch) filter to suppress the desired transmit frequency in your product , in order to measure any spurious signal generated by your circuit.

Other times, you could also insert it after the upconverter mixing stage to suppress any LO leakage to the RF port.

as every one else have said here the Band stop filter is a filter inverse to the Band pass filter and it stops a particular band of frequencies; The notch filter is a special case of a band pass filter with a sharp cut-off; these filters can be made electronically tunable and are mostly used in Electronic warfare systems and also to suppress harmonic levels in systems requirng a high degree of spectral purity.