SEOmoz is a web app and tool suite that allows users to analyze websites for SEO opportunities and issues. The main features covered in the document include crawling websites to find errors and missed opportunities, tracking keyword rankings and traffic over time, competitive analysis, on-page optimization reports, link analysis, and integration with Google Analytics. The document provides examples of how SEOmoz can be used to diagnose crawl issues, analyze duplicate content, check for canonicalization problems, monitor keyword rankings, and view key link metrics for a domain.