Windows

Put simply, brick to brick removes the entire original window structure out to the masonry opening in the building envelope, and replaces the entire thing. It means, among other things, that all of the window casings, inside moldings, window sills, etc. will need to be replaced as well. Messy and expensive, but provides many more options, an opportunity to improve the insulation, better fit, and it may be that the existing window frames are not salvageable.
A within frame installation keeps the existing perimeter window frames, and replaces the glass units and their immediate frames. If you think of a typical double hung windows, the two existing moveable rectangular window units are removed, and a new window installed in that opening. Among other things, this method may shrink the glass part of the window, and thereby slightly reduce light/visability, and in some cases the retrofit is obvious. It is, however, cheaper and more convenient, especially if not part of a larger renovation.

We are in a landmark district and needed to replace windows that were in bad shape. Some did not open because of broken mechanicals, others had compromised seals. After a whole lot of back and forth with Landmarks, we were faced with the option of either replacing the whole window (14 of them) with beautiful brick-to-brick, wooden windows to conform with what Landmarks felt was historically appropriate, or simply replacing the glass and repairing the mechanisms in our ugly, non-conforming existing windows. The cost of new windows was in the $50 thousand+ range. New glass in old windows was $15 thousand. Guess what we did.

Thank you both for your comments.
@Joandark, what were estimates for pricing of both options? How many windows did you replace? Did you have to put up scaffolding? Any links to the options online?

We replaced the glass and repaired 14 windows. All work was done from the inside.