Online dictionaries
NEWBURY HOUSE ONLINE DICTIONARY BY HEINLE & HEINLE http://nhd.heinle.com This has one short definition with a sample sentence. I recommend starting here when you are learning words. Then move on to a more sophisticated set of definitions.
CAMBRIDGE DICTIONARY Students like this site because it explains HOW to use the word and gives multiple example sentences. Its easy-to-understand definitions are student friendly. I frequently use versions of the Cambridge dictionary sentences on quizzes. http://dictionary.cambridge.org/default.asp?dict=A
WORD NET-- PRINCETON'S ON-LINE DICTIONARY http://www.cogsci.princeton.edu/cgi-bin/webwn This site can give you loads of context clues. It was developed by cognitive scientists at Princeton University based on research about how people learn words. For an overview of the site visit: http://www.cogsci.princeton.edu/~wn/
MIRRIAM-WEBSTER DICTIONARY Writers of the WordMaster Analogy test use Webster's dictionary for their definitions. Here's a link to the Merriam-Webster online dictionary: http://www.m-w.com/home.htm. This site has brief, easy to understand definitions which explain where the word comes from (this is called the etymology of a word.)
DICTIONARY.COM http://www.dictionary.com/ Look for sample sentences at the end of definitions. These definitions tend to be more detailed (and sometimes more adult) but, the advantage is that these definitions have lots of connotation and context info in them.