For those of us who mainly get on the internet to read Freerepublic and e-mail, all I can say is; "Huh? What are you talking about?
Thanks for asking and I'll cover everything point by point.
1). One may use a "User Agent" setting so that regardless of the operating system or web browser, it can masquerade as something else. My bank and credit card companies will allow only a Windows or Mac to access their web sites. So I have to "fake" my platform's identity so that it appears to be Windows. Frankly, this is often due to sloppy web design since the underlying client operating system is of no consequence. The web was designed to be platform neutral so, yes, it does irk me that I have to masquerade the fact that I use Linux.
2). As for the reference to "nginx". That's the webserver software used by Free Republic. A few years ago, they used "Apache" but for reasons I'm not privy to, Jim and/or John changed to "nginx". There's also a database backend that powers Free Republic but I'm not certain what it is, probably MySQL since that's the one widely used on Linux.
3). The access log is maintained by the webserver software. Every single connection is logged with a timestamp, the incoming IP address, the page being viewed and the "User Agent". The latter field in the log allows one to discern which web browser and operating system is used by the incoming connection. So if Jim and John are curious and scan the logs, for my connection, they would be misled into thinking that I'm using Internet Explorer and Windows. The reality is that I'm using the Midori browser and Linux.
In short, those of us with a Linux desktop system who can't (or don't want) to run Internet Explorer have to fib about out identity so numerous sites won't reject us. I could take the trouble of masquerading my identify on a site-by-site basis but it was just easier to set it up globally for all sites.
So there you have it -- a lengthy reply and I sure hope it covered everything you wanted to know. If not, just ask for a clarification. As should be apparent, I enjoy talking about this stuff. :)