Punctuated equilibrium refers to the theory that there were long periods of slow evolution interspersed with short bursts of evolution. These short burst might be explained by a need to adapt quickly to a changing environment.
Gradualism is another form of evolutionary theory where evolution takes place slowly over long periods. Small selection pressures will cause change over long periods.
With punctuated we know that sometimes the environment changed drastically over a 'short' period of time. This could explain why we see very similar organisms in the fossil record with a sudden 'new' form appearing.
