This type of visa will allow you to obtain the Green Card for you, your spouse and your unmarried children under 21 years old.
If you are an extremely talented alien, you may qualify for this visa. People who qualify for this visa are usually at the top of their field. There are no restrictions on the type of fields eligible for this type of visa.
People who qualify for this type of visa are given special preference. Therefore, unlike many other types of visas that take years to obtain, this one is given special priority.