If the vowel (in this case "a") says it's name, it is consider a long vowel. Such as in the word "take". If the vowel does not do that and make another sound, it's considered a short vowel. Many people don't understand the importance of understanding long and short vowels. It becomes a very valuable skill once you start to encountering higher level vocabulary words.
The most common long a words are: baby, lady, paper, bake, rake and lane. The most common short a words are: dad, hat, man, sat, fan and lane.