I'm on Mac OS X 10.11 El Capitan right now and there is no such choice of buttons (i.e., between Names and Contents). If Contents was selected, then Finder would look inside files for the text that is typed into the search bar. In Mac OS X 10.5 Leopard, and possible Mac OS X 10.6 Snow Leopard, there were two more buttons all the way over to the right that said Names (or maybe it was Filenames) and Contents. Starting from the left side of this bar, there is the text Search: and next to it there are two buttons that say This Mac and the name of the current folder in double quotes. In Mac OS X 10.5 Leopard through at least Mac OS X 10.11 El Captain, when you typed in the search box in Finder, a silver bar would appear below it.
