He has also suspended the US Refugee Admissions Program for 120 days until it is reinstated “only for nationals of countries for whom” members of Trump’s Cabinet deem can be properly vetted.
Nobody from any of those countries has committed a terrorist attack on US soil. It is unclear what this ban will achieve.
The largest terrorist attack on US soil was committed by citizens of Saudi Arabia. Saudi Arabia is not on the banned list.
This will affect refugees from those countries. Ironically, Trump signed the order on January 27, also known as International Holocaust Day. The Holocaust would have been a lot worse if the US had not admitted many Jewish refugees to settle in America.