The above responses are correct.
To add a little detail that many are not aware of: the $18,000 limit is for "elective deferrals" only. If your retirement plan has a mandatory contribution component (mine does, but it's a 403b so not sure if the same can apply to a 401k), that mandatory amount does not count against the elective deferral limit. As a result, I will end up contributing about $20,500 to my 403b this year, in addition to my employer's contributions of about $13,000.
Edit to add: The first comment
here explains the details behind this nuance, in case anyone is curious.