The first link is the Crayola (as in crayons) site and it has this info about long and short vowels:
The English vowels a, e, i, o, and u make different sounds in words. When these letters seem to say their own names, they are often called long vowels. You can hear long vowels in these words: game, heat, life, rope, and cute. Vowels can also make short sounds. You can hear them in hat, pen, gift, hop, and run.
The second link is to Wikipedia's entry on long and short vowels. It says the says the same thing: a vowel's long sound is it's name, and short vowels are vowels that do not sound like their name.