When non-biologists talk about biological evolution they often confuse two different aspects of the definition.  On the one hand there is the question of whether or not modern organisms have evolved from older ancestral organisms or whether modern species are continuing to change over time.  On the other hand there are questions about the mechanism of the observed changes... how did evolution occur? Biologists consider the existence of biological evolution to be a fact.  It can be demonstrated today and the historical evidence for its occurrence in the past is overwhelming.  However, biologists readily admit that they are less certain of the exact mechanism of evolution; there are several theories of the mechanism of evolution.1
Punctuated equilibrium is a theory about how new species evolve that was first advanced by American paleontologists Niles Eldredge and Stephen Jay Gould in 1972.  Although controversial, punctuated equilibrium has stimulated fruitful debate about speciation (the birth of new species) and the fossil record and has, in recent years, won at least partial acceptance among most evolutionary biologists.2
Before punctuated equilibrium, most scientists assumed that evolutionary change occurs slowly and continuously in almost all species, and that new species originate either by slow divergence from parental stock of sub-populations or by slow evolutionary transformation of the parental stock
itself.2 
Evolution: a fact and a theory
Punctuated equilibrium proposes that most species originate relatively suddenly (over tens of thousands or hundreds of thousands of years, rather than the millions of years assumed by traditional theory) and then do not evolve significantly for the rest of their time on Earth.  Most species thus have a sudden or punctuated origin and then remain in stasis or equilibrium until extinction.2
Eldredge and Gould proposed punctuated equilibrium to explain one of the most notable features of the fossil record: most species seem to appear suddenly, already clearly differentiated from the earlier, similar species from which they presumably evolved, and then remain unchanged until becoming extinct.2
Traditional evolutionary theory proposed that gradual evolutionary changes are rarely observed in the fossil record because that record is radically incomplete. Fossils form only under certain special conditions, fossil-bearing rocks are eroded as well as deposited, and our knowledge even of those fossils that have been formed is fragmentary.2
Eldredge and Gould agree that the fossil record is incomplete, but contend that it could not be incomplete enough to account for the near-complete absence of gradualistic change in the fossil record.  Rather, they propose, species normally originate too quickly for normal geological processes to record the event.2
1- Evolution is a Fact and a Theory
2- Science Encyclopedia