AboutSWEC (Simple Web Error Checker) is a program that automates testing of dynamic websites. It parses each HTML file it finds for links, and if those links are within the site specified (ie. local, not external), it will check that page as well. In this respect it works a lot like a crawler, in that it'll click on any link it finds (more notes about this later).
In addition to parsing and locating links, it will also parse the pages looking for known errors and report those (such as Mason or PHP errors), and will report if a page can not be read (by either returning a 404, 500 or similar).
Since you may often want SWEC to be logged in on your site, you have to be careful. When logged in, SWEC will still click on all links it finds, including things like 'join group' or 'delete account' (though it has some magic trying to avoid the latter). Therefore it is highly recommended that when you run SWEC as a logged-in user on a site, use a test server, not the live one.
Running SWEC on a live site without being logged in as a user is perfectly fine, it won't do anything a normal crawler wouldn't do (well, not exactly true, SWEC will ignore robots.txt).