For the uninitiated, the special thing about the Oxford English Dictionary is that it is not really a dictionary in the sense you are probably used to. It has definitions but they are not the focus. The heart of the OED is the etymologies - they are incredibly thorough and they are accompanied by citations (example sentences) that show not just typical usage of the word like modern dictionaries, or a particularly distinguished use like Webster's, but a series of historical examples beginning with (often) the earliest known written use of a word and tracing its evolution to the present day.

There's something special about the printed OED but the online version is far easier to use, and also up to date. It is an expensive annual subscription ($250 or so) but you may have free access via your local library.