There are loads of really good sites out there, full of information about trees and how to identify them.
http://www-saps.plantsci.cam.ac.uk/trees is one such site which allows you to identify a tree based on its leafy twig or a winter twig. It also lists tree by name, helping you increase your existing knowledge
Stihl – http://www.stihl.co.uk – also have an encyclopedia of trees. It is located under the ‘Know How’ menu. It provides a lot of information is a user friendly format but does not appear to help much if you do not know the name (or part of it). It also is the only site listed here to contain an encyclopaedia of tree diseases.