Cardinal grapes are  large sized grapes that have seeds, and have firm skin, and flesh, and  super sweet flavor. When ripe,  Cardinal  is a dark black/purple color. The bunches should be long and full of grapes. This type of grape is very popular in the  United States and is used extensively in foodservice. 

As with all grapes,  Cardinals are low in sodium, high in anti-oxidants, have food amounts of fiber, and are low in calories, with 70 per cup. Grapes do not get sweeter after picking, so color is the best indicator for ripeness and sweet flavor. Look for grapes that are plump and firmly attached to pliable stems. Avoid soft or wrinkled fruits.

Grapes are sold ripe and ready to eat. Refrigerate unwashed. They will keep will for up to 7 days at 38-40 degrees F. Just before service, rinse grapes with cool water. 

One pound of seedless grapes with yield 3 cups.

Here are three in-depth and excellent grape sites for information, recipes and more:

http://www.tablegrape.com/rev99/index.htm

http://www.dole5aday.com/encyclopedia/Grapes/grapes_types.html