One of several installation methods may be more appropriate for your situation than another. There is always a cost trade-off so you need to know exactly how a contractor plans to approach your job and why a particular installation method is being used. Factors that determine which installation method is to be used include the frame construction of window that is being removed, the frame construction of the new window being installed, and the personal preference of the owner.
WOOD (type of frame coming out)

Sash - Is the moveable (typically) part of the window surrounding the glass.  The sash fits within the frame of the window.
Frame - The part of the window that is attached and sealed to the structure of the house.

Block Frame:
Install an "Insert Window"

Pros Cons
Maintains integrity of the flashing and existing window frame/sill, does not disturb interior or exterior trim, minimal dust, fast and easy. Small reduction of glass area

Remove the old sash and replace with new sash
Pros Cons
Can be exact duplication

Simple Installation

Double-hung has a vinyl jamb liner

Relatively expensive window


Glass Set in Place:
Remove old glass & install new glass
Pros Cons
Keeps clean, open look

No reduction of opening size

Warranty on glass is limited to 10-years

Out of square openings require additional trim work.