The advantage with using log files to collect data is, that you don't need to add any code to your pages and that all your pages are automatically analyzed.
In One.com Statistics we define the following terms.
Request - A request comes from a browser to open a file on your site. Usually a page on your website consists out of different files, like images, text and css. So opening one page on your site, will generate several requests. In the One.com Statistics we use requests to measure the most popular URL's.
Visit - A visit is a continuous browsing event with no more than a 15 minute break between requests. The requests must come from the same browser and the same IP.
So, as long as a visitor clicks somewhere on your site every 15 minutes, it is counted within the same visit. If the visitor doesn't do anything for more than 15 minutes and then clicks again, this is counted as new visit.