A PI filter is a filter that has a series element and two parallel elements connected in the shape of the Greek letter PI.
Butterworth filter are characterized by a constant gain (flat response) across the midband of the circuit and a 20 dB per decade roll-off rate for each pole contained in the circuit.
Butterworth filters are termed maximally flat magnitude response filters, optimized for gain flatness in the pass band. The attenuation is -3db at cutoff frequency. Above the cutoff frequency the attenuation is -20 db.
The Butterworth is the only filter that maintains same shape for higher orders whereas other varieties of filters (Bessel, Chebyshev, elliptic) have different shapes at higher orders.
A Low pass filter is a filter that passes low-frequency signals but attenuates signals with frequencies higher than the cutoff frequency.