Trump's "Muslim Ban" has begun, in the form of a ban on the entrance of nationals from 7 Muslim-majority countries (Sudan, Libya, Yemen, Somalia, Iran, Iraq and Syria).
Read more: BBC.
A discriminatory and arbitrary measure that goes against the fundamentals of the United States.
Muslim terrorism is not currently a threat inside the US, and the way of stopping it is not banning anyone from specific countries. Vetting processes to detect dangerous people applying for visas are already in place, and work.
The most unfair part of the policy is that some big Muslim countries are excluded, key American allies like Saudi Arabia and Pakistan. Countries whose nationals have actually attacked the US and funded Islamic terrorism.
You can read the Executive Order yourself, here (pdf file).