A tree's leaf is one major marker that helps in keying out and identifying any species of tree. Most trees can be identified by the leaf alone.
As you can see in the illustration, leaves come in many shapes and sizes. The "star" shape of sweetgum is totally different from the heart-shaped leaf of an eastern redbud. Note that leaves can be described by observing their base, their margin their veins and their tip or apex. Each characteristic has a name and is used a part of the identification process.