The application fingerprinting is based on checksums and string matching of known files for different versions of CMSes. This results in a score being calculated for each detected CMS and its versions. Each detected CMS is displayed along with the most probable version(s) of it. The score calculation is based on weights and the amount of "hits" for a given checksum.
wig also tries to guess the operating system on the server based on the 'server' and 'x-powered-by' headers. A database containing known header values for different operating systems is included in wig, which allows wig to guess Microsoft Windows versions and Linux distribution and version.