By default, Hextrakt keeps the last 5 crawls in the history. You can change this in the crawl configuration (perimeter tab). To compare crawls efficiently, preserve the same crawl settings / perimeter for all your successive crawls. If you crawl big websites, check that you have enough disk space available.
To compare with a previous crawl, select the crawl date in the list. The history comparison report show all new errors, new warnings, new found (HTML) pages, new URLs, lost URLs.