Say you'd like to find out what a "clew" is. You could search for define clew, and most of the search results would have some sort of definition. However, this is really just a keyword search, so some of the results might be long articles on clews or only mention the definition in passing.
Define: Your Terms
If you're really only interested in finding a quick dictionary style definition of clew, use the syntax define:. The search in this case would be define: clew. From that search, we can instantly see that a clew is the lower corner of a boat sail.
The information is coming from a variety of dictionary related Web sites, and there's a link to the full entry for each Web site. Google also provides links to related searches, such as "clew bay."
What If You Can't Spell?
If you aren't the best speller or you make a typo, don't worry. Google will still suggest an alternate search, just as it does for regular Web searches. If we type in define: cliw, Google helpfully asks "Did you mean: define: clew."