"Crawling is the process of an engine requesting -- and successfully downloading -- a unique URL," he says. "Indexing is the result of successful crawling. I consider a URL to be indexed (by Google) when an info: or cache: query produces a result, signifying the URL's presence in the Google index."
Both are vital to maintaining a successful Web site (and snagging high organic rankings), but one of the differences stems from the amount of time each takes to get done. A site can be crawled within hours of being uploaded to a domain, but it can take much longer (in an example Dafforn provides, more than a week) to be indexed.