What are the differences between Crawler impact rules and crawl rules in SharePoint 2010 search?
Crawl rules define the "what". You can include/exclude the content you want to crawl. What urls, pages, documents, images etc.
Crawler impact rules defined the "how" or "when". You can control how often the crawl service will request content. So for example, if you only have one server, you could defined impact rules to reduce load on your servers. If you have dedicated crawl/index servers you can request content more frequently instead.