The Font window in Cocoa applications such as TextEdit, Pages, and Mail is a floating window that gives you access to all of your machine’s installed fonts. It’s typically mapped to the Command-T keyboard shortcut. When the Font window is activated, it doesn’t take the focus from the window used to activate it. In general, this is a good thing, as it means you can keep typing after the Font window appears. The Font window won’t even take focus if you click on any of the font entries. Again, this is a good thing, as it means you can change your document’s fonts without having to toggle the focus between the Font window and your document’s window. However, it also means you can’t use the keyboard to select font collections, families, typefaces, or sizes—because anything you type will be sent to the document window, not the Font window.
If you’re experimenting with a number of fonts, however, you might want to use the keyboard to quickly scroll through them all while seeing how the each font looks with your chosen text. To do that, you need to switch the focus to the Font window. There are at least two ways to do this that I’m aware of: Click on the Font window’s title bar (where it says Font), or click in the Search box at the bottom of the window. When you do either of these things, you’ll see the pattern of the window’s title bar change to solid gray, indicating that the Font window now has the focus. You can now click into the Collections, Family, Typeface, or Size columns, and they’ll become the active selection. (Notice that the selected item in any of those columns is highlighted in your system’s highlight color, not the gray color that you see when the window isn’t frontmost.)
Once you have a column active, you can use the up and down arrows to cycle between the choices within a column, and you can use the Tab key to move the highlight to the next column. Unfortunately, you can’t select a font by typing a few letters of its name, as you can in many other list-like views in OS X.
When you’re done with the Font window, you can make it vanish again by pressing Command-T. Alternatively, if it’s still the frontmost window (i.e. has focus), you could also use Command-W.