It is easy to embed Tcl into a legacy program so you can script the behavior of that program and add in other building blocks like a GUI interface. Tk is a window system toolkit for building graphical user interfaces. Tk makes GUI programming very easy, but at the same time it offers fine grain control and some sophisticated widgets so you can create great interfaces. In particular, its text and canvas widgets are very powerful. The text widget supports multiple fonts through a tag facility.
Tags apply to ranges of text and can have different appearances as well as event bindings.
Canvas widgets provide display-list graphics so you can efficiently manage thousands of lines, rectangles, ovals, images, text items, etc. The canvas also uses tags so you can easily add behavior to these light-weight items. It introduces the notion of objects, which act as building blocks for an application.