What kind of tree is this?  That is a question faced by someone when encountering a unidentified species of tree.  Trees are identified by their leaves, bark, branch pattern, buds, flowers, and fruits.  There are two tree guides commonly available in the local bookstore that are a good reference to identify trees.

• National Audubon Society - Filed Guide to Trees, Eastern Region, North America, by E. L. Little, (1980)- revised 2000

• A Field Guide to Eastern Trees (Peterson Field Guides) 
  by George A. Petrides, et al (Paperback - July 15, 1998) 

Each of these books has it strengths and weaknesses, but are good guides for for the tree enthusiast.  The Audubon book is more visually oreiented with hudereds of photographs.  The Peterson Guide has more illustrations and is orginized more like an identification key.  Even with these guides it may be difficult to determine the species of a particular tree. Don't feel bad, even the experts are constantly debating the identity of difficult specimens.  Some trees, like oaks, may hybridize between similar species producing a tree with characteristics different from either species alone.  The American Chestnut Foundation in Pennsylvania asks that people reporting large American chestnut trees to them, to send a leaf sample to help determine if it is a true American chestnut or one hybridized with introduced Chinese Chestnuts.  In addition to these books aimed at the general public there are a large number of forestry textbooks and papers dealing with tree identification.

Many states and organizations have online tree identification tree guides that lets the user work through an identification key.  A search from any search engine will locate dozens of them,  http://www.metacrawler.com Three sites in particular are worth visiting:

• Virginia Tech University:   http://www.cnr.vt.edu/dendro/dendrology/idit.htm 

• The National Arbor Foundation:  http://www.arborday.org/trees/treeID.cfm 

• Gynosperm Database http://www.conifers.org/