The FontDialog in Tycho is a top-level window within which you can select a font. You will usually get access to it through the preferences manager.
The window contains three option menus which you can use to set the font family, size, and style. The "Screen" family is a psuedo-family that maps to some common X fonts such as 8x13 and 9x15. There is a display of text in the selected font, and, for the hackers, the name of the actual X font selected. When you make a change to the selected font, the displayed text changes to that font.
The font dialog (and the class that is doing the real work behind the scenes, the font manager, hide the arcane font specification that is used by the X window system from the user. As part of this, the font manager will always choose a valid X font, regardless of the specification you give it. For example, if you try and select a "Screen" font in italics, the font manager will display a Courier font since there is no italic screen font.