Terrorism has been a fear around the world for many years, but the United States always held a somewhat complacent view: "It Won't Happen Here." Tragically, the events of 9/11/01 showed this view to be entirely wrong, and since then the U.S. government has been taking steps to prevent future attacks.
The most obvious change has been in transportation security. The Transportation Security Administration (TSA) was created after 9/11 and has been given great leeway in developing and implementing methods of passenger screening, especially on airplanes; these policies are controversial as many believe that profiling would allow greater security than so-called "nude scanners" and the extremely unpopular public patdowns administered at random. However, while the increased screening processes have not, as of today, prevented any attempted attacks, the increased security on the actual airplanes have resulted in several failed bombing attempts, such as the Underwear and Shoe Bombers, both of who were foiled by passengers and Air Marshals.
Border security on land has been slightly more effective; since the majority of illegal alien and drug transportation takes place on public roads, increased suspicion and verification of documents has been able to stem at least some of the illegal activity on the borders.
Finally, the intelligence-gathering arms of the government have been stepped up (e.g. the Patriot Act), allowing surveillance and apprehension of major terrorist figures around the world. These new policies are seen by some as the forerunner to a police state situation, but by others as necessary reactions to the changing world.