A cap sleeve is one which basically covers the very upper arm and shoulder.
They tend to be shaped from the under arm out in a curve to a point no more than 40mm below the tip of the shoulder.
T-shirt
Half way between cap sleeves and short sleeves (see below), these sleeves will end half way between the shoulder and the elbow
Short
Reaching the elbow, short sleeves are a life-saver for anyone living in fear of their upper arms. Having said that, we're sure that the guests will be looking at the Bride's radiant smile, not their arms!
The short sleeve option is a traditional one common in formal Weddings.
¾ Length
Again, relatively straight forward, ¾ length sleeves stop half way between the elbow and the wrist and hence cover three-quarters of the arm.