Here's a UK-based site that does a nice job of "tracing" (ha! get it?) the functional and artistic changes to the gothic window style from Perpendicular (mostly UK) through the high gothic period. There are some nice modern line drawings that help illustrate the styles that evolved over hundreds of years and great distances apart from each other. For the architecture geek: Lookingatbuildings.org.
Bishop's Eye Window, Lincoln Cathedral
Some serious design challenges, are met and conquered, in the project of forming a rose window (circular) built from curvilinear tracery (cement framework that holds the glass in place.) They did it by first breaking the circle into manageable interlocking sub-forms, and then rebuilding the design into a cohesive whole. The repeating sub-forms or shape themes, and color motifs, give the rose windows strength and satisfying unity. These were pretty difficult design challenges, and not for the weak at heart; that's why it's pretty rare for a cathedral to have one. Lincoln has two, the Dean's Eye, and the Bishop's Eye. (at right)
Dean's Eye Window, Lincoln Cathedral
Most rose windows are based on a radial design, meaning the design repeats symmetrically, radiating out from the center. The Dean's Eye is a good example (right). The Bishop's Eye (original from the year 1220, rebuilt in 1330s) is unusual in its interlocking leaf forms-- that are symmetrical but not radial.
See more great pictures of Lincoln Cathedral. Plan a trip, maybe.