When ever you have a vowel that sounds just like its name we call it a "Long Sound" or "Long Vowel". Look at the word "Kite". Sound it out. The "i" sounds just like the vowel itself. Long "i"s can also come from the letter combination of i_e, igh, le, and plain old -y. Such as in the word "fry". Cool fact the word "set" has the most definitions of any other word in the English language.
The most common long a words are: ice, five, bike, mice, nice, nine, side, wide and fine. The most common short a words are: big, did, his, lip, pin, pig, sit, sip and zip (had to get a "z" in there!) Scroll down to print out some great worksheets to help you determine the differences.