Hits are different from visits because one person might load the same page multiple times within the same visit to the site. A "hit" is counted any time the page or file is accessed for any reason - including being crawled by bots, etc. Whereas a "visit" implies that somebody's actually at the site and all "hits" on that page during the person's visit are grouped into that visit. Also, hits are counted any time ANY file is accessed... an image file, sound file etc.
"Total files" just means how many files were downloaded - not just HTML files but images, MP3's and anything else embedded in the site.
"Total pages" is the number of actual pages... HTML files or scripts that generate HTML pages.
"Total KBytes" is just that... total number of kilobytes downloaded. Useful if you are being charged for excess bandwidth.
Total Unique Sites, Total Unique URL's and total unique referrers have to do with the sites where your visitors came from - who has linked to you or the last site they visited before yours.
The User Agent is the browser - total unique user agents is the total number of unique browser types/versions that have visited you.
Hope that helps!