Trump signed an executive order targeting skilled foreign workers - here's why the H-1B program is so controversial

President Donald Trump signed an executive order on Tuesday directing federal agencies to review and propose changes to a popular, but controversial visa program meant to attract skilled foreign labor.

The H-1B visa program has been assailed for years by critics who say it is used by companies to hire cheap, foreign workers in place of Americans.

Its proponents say it provides much-needed skilled workers to sectors where companies have struggled to hire Americans.

"Jobs must be offered to American workers first. Does that make sense?" Trump said Tuesday during a visit to a tool manufacturing company in Kenosha, Wisconsin.

"Right now, widespread abuse in our immigration system is allowing American workers of all backgrounds to be replaced by workers brought in from other countries to do the same job, sometimes for lesser pay."

Here's what the program entails, and what the implications of Trump's order are: