Simple php if else on one line question

Hey guys, I’m guessing this is a really easy one but I can’t figure out what this is called. I’m looking at someone elses code and here’s the snippet:

$id = ($this->uri->segment(3)  == "") ? '16': $this->uri->segment(3);

The whole ? and then : thing, what’s that called and where can I find more information about how that works? thanks!!!