You will often hear contractors and manufacturers use the terms New Construction or Replacement (Retrofit) when discussing residential windows. While neither window must be used exclusively for the project after which they're named, each has specific advantages designed for these different installation methods.
New construction windows typically have a nail fin. When homes are built, the framing is likely constructed of wood studs. The nail fin rests against the outside of the stud, and nails or screws are driven through the holes in the fin. All other exterior materials are then applied and butted up to the frame of the window or trim.
New construction windows also can be used in a replacement project. The exterior materials, such as window trim (if it exists), siding, stucco or flashing, are removed, exposing the wood studs. The new window is then installed as it would have been when the home was originally built. Exposing the original studs also allows your contractor to see potential damage (typically rot) or home settling that may have occurred. The damage can be repaired, and the new window installed with proper insulation to minimize air infiltration between the new window and the studs. As you can imagine, this method can be more time consuming so often costs more than other replacement methods.