First, the Oxford English Dictionary (OED) is a really cool dictionary. It's huge (many many volumes long), but that isn't the only reason it's great when you can't find a world in smaller dictionaries. Most dictionaries are of current usage: they'll give a definition of a word that is pretty much how it is used today. The OED is a historical dictionary, covering the current AND outdated meanings of words, with the context they were in at the time in the form of quoted historical text. Why is this good? For those times you are reading an old book and need to know what a word meant back then! It's also updated on a regular basis, so it gets the new words much faster than you'd usually buy a brand-new dictionary.
Second, having the OED available in an online form makes it even more useful. Not only do you save the shelf space all of those books would take up and have access to it after the library is closed, you can search it in ways you never could with the paper-and-ink format. Would you like to find out what words were first spotted in the wild in a particular year? Search for "first cited date." Want to find out what words were first spotted in writing by a particular author or in a particular book? "First cited author" and "First cited work" will do that. You can work your way to just the right word by searching for terms in the definition. How about all words derived from a particular language? You essentially have a full set of dictionary power tools.
And the best feature of all, the "Lost for Words?" button that will give you a random dictionary entry!
