A tree key is one of the most helpful tools a dendrologist, forester or amateur botanist can use to identify a tree. I have created a very simple tree leaf identification system for beginners.
Tree Finder - A Quick and Easy Way to Identify 50 Common North American Trees
Keys can use leaves, twigs, flowering parts and other tree characteristics to narrow down an unknown tree to a final classification. Some keys use terms that are a bit technical and a glossary is usually included to define terms mentioned in the key.
I have created a handy FAQ on how to identify trees. Start here and take my Tree Identification eCourse:
Tree Identification and Dendrology - FAQs
Beginning Tree Identification eCourse