Windows are an integral part of your new home and it is important to understand the differences.
A single hung window is a more affordable window that only has one workable sash (meaning only the bottom part goes up). Usually this type of window does not have wooden trim surrounding the window and they will have a sill at the bottom that is the total trim package. These windows work fine but are of a lower quality. You will see single hung windows in many affordable multi-family housing offerings and more affordable single-family homes and you can pick up on them easily as sheet rock walls surround the window.
Double hung windows (meaning both sashes can be operated) allow for wooden trim all the way around the window giving a much better look. Many double hung windows have the ability to tilt in for ease of cleaning, etc. A double hung window comes in many varieties, styles and level of quality. Some of the upper-end double hung windows are manufactured locally allowing the consumer to get all of the features provided by the national brands at a much lower price.
Check with your builder prior to construction to find out what type of windows and features these windows will offer you. Windows are an expensive item and you want to make sure that the ones in your home will last and provide the look you expect.