Generate a butterworth filter.
Default is a discrete space (Z) filter.

[b,a] = butter(n, Wc)
low pass filter with cutoff pi*Wc radians

[b,a] = butter(n, Wc, 'high')
high pass filter with cutoff pi*Wc radians

[b,a] = butter(n, [Wl, Wh])
band pass filter with edges pi*Wl and pi*Wh radians

[b,a] = butter(n, [Wl, Wh], 'stop')
band reject filter with edges pi*Wl and pi*Wh radians

[z,p,g] = butter(...)
return filter as zero-pole-gain rather than coefficients of the
numerator and denominator polynomials.

[...] = butter(...,'s')
return a Laplace space filter, W can be larger than 1.

[a,b,c,d] = butter(...)
return  state-space matrices

References:

Proakis & Manolakis (1992). Digital Signal Processing. New York:
Macmillan Publishing Company.