If Wikipedia can get it right...
There seems to be an ever growing alarming mass of misinformation about evolutionary theory being spewed onto the internet, and into public school systems world wide... how then can such a widely criticised "knowledge by democracy" manage to get the job done so well. Just take a look on the article series on Speciation.
The diagram (taken from the speciation article series) on the left illustrates quite plainly and simply the four major known and demonstrated means by which speciation can occur.
If you feel confused about evolution, take a wander through the Speciation series at wikipedia. It's really good. Like text book good (maybe better!).