User Agent strings vary; usually, when viewing server logs, by and far, the most common User Agent string is:
Mozilla/4.0 (compatible; MSIE 6.0; Windows NT 5.1)
Microsoft Internet Explorer 6 running on Windows XP. The Mozilla section is there to be compatible with legacy Internet sites that refuse to serve content to non-Mozilla browsers.
Some User Agents are a bit more detailed than others; Mozilla's UA is a good example
Mozilla/5.0 (Windows; U; Windows NT 5.1; en-US; rv:1.0.0) Gecko/20020530
Not only does it show what version of Mozilla you're running, it also details if you have strong encryption (the U), what locale you're using (en-US), the revision (v1.0.0), and the build date (Gecko/20020530)
Some sites (online banking, mostly) nowadays will reject you if you're using a non-IE browser. The thinking behind this is that since MSIE has ~95% of the market, no time and effort should be wasted coding for browsers only 2% of people use. This is stupid. Ironically, many websites will allow Netscape 4.x, which is notorious for its CSS/DHTML incompatibility in, while rejecting Netscape 5.x, which renders far better. Many tools exist for changing your User-Agent header for such situations, or whenever you want to protect your privacy.