Keep a window always on top in Mac OS X
Yes and no: Yes, Mac OS X does have this ability for a window to always be on top (aka floating windows) but no, it's not built in to allow you to do that to any window as the user. However programs such as Afloat can help do exactly what you want to do.
In response to the comments on the answer, Install guide is here. Apparently it still works with yosemite.
(I do not have enough rep points to comment, so....)