Most failures are because a given HTML or graphic file could not be found. This is an approximate number of individual people that visited your site. Specifically this is the number of unique internet addresses that accessed your site. It is common for many people to be behind a single internet address and it is possible for the same person to access your site from different internet addresses. So this is only an approximate number.
Pages are HTML files.
Requests is the total of HTML files, pictures and any other file. This shows from which top level domains your visitors came. This is the best place to look for international visitors. This shows which domains came to your site most often. This is the most detailed of the domain reports.