Basically all windows can be considered replacement windows. The two main differences are "Insert" style and "Full Frame" style. Insert is just as it sounds, the sashes and track come out and a whole new custom sized window goes into the existing frame. Full Frame means the entire window, frame, casing and all are removed and a new window unit is installed with new interior casing as well. The most common "Insert" window is a vinyl window, however, most manufacturers of wood windows also offer a custom sized insert window. "Full Frame" windows for the most part are wood frames and sashes with either aluminum of fiberglass cladding on the outside to protect the wood from the elements.