You may have heard of talk about H.R. 158, the "Visa Waiver Program Improvement Act of 2015" (which may also be referred to as the "Visa Waiver Program Improvement and Terrorist Travel Prevention Act of 2015"), which has not been passed by Congress, but you are really looking at the wrong bill. This bill has not passed because it is no longer being considered. Congress decided to pass the provisions through a different bill because politics is confusing like that.
Changes to the Visa Waiver Program have indeed been recently passed, as part of H.R. 2029, the "Consolidated Appropriations Act, 2016" (the "Omnibus" bill), and was signed into law by the President on December 18, 2015. The changes to VWP are in Division O, Section 203 of the bill. The gist of it is that people who have traveled to Iraq, Syria, Iran, or Sudan since Mar 1, 2011, or who have the nationality of Iraq, Syria, Iran, or Sudan (regardless of whether they have ever been to that country), are disqualified from using the Visa Waiver Program.
People who don't use the Visa Waiver Program are not affected by these changes. (For example, some countries are not part of the VWP but their nationals can visit the US without a visa: e.g. Canada, Federated States of Micronesia, Palau, Marshall Islands, and, obviously, the US.)
Update: The US announced that it will begin implementing the changes on January 21, 2016. It says that people who currently hold ESTAs who have indicated having one of those 4 nationalities will have their ESTAs revoked. It says that the Secretary can waive the restrictions for certain reasons but it's not clear where in the ESTA process such a waiver would be applied for.