That’s a fascinating question! I’m not a biologist, but I’ll give it my best shot to answer. From what I can tell, it’s a subject that causes a lot of arguments among the scientists who study this.
The argument for those who say that evolution has stopped for humans goes something like this – evolution happens because the organism has some good characteristic that makes it more likely to survive long enough to reproduce. In this way, the good characteristic is passed on to future generations, while other organisms that don’t have the characteristic are less likely to be able to reproduce. So the organism over time evolves to have the good charateristic. In modern society, medicine is so advanced that almost everyone is able to live long enough to have children, so some scientists argue that this means that new characteristics are no longer being selected.
Other people disagree, saying evolution is now mainly happening in our brains, and that there are changes being observed in the human population. Also, a dramatic event like a huge new disease might also cause evolution in humans, as those who can resist the illness will be able to reproduce.
It would be amazing to be able to fast-forward and see what humans are like millenia from now (if we still exist at all!!) 🙂