You login to a mysterious new box. There is no login message. You poke around and before long you start to wonder “So what the heck distro is this anyway?”
$ uname -a
just tells you all about the kernel. Hmmm. A mystery.
To pull up details on the distribution, take a peak in
/etc/issue. This text file is often what is presented to users after they login, and typically contains distribution specific details. Likewise, look for
/etc/*version, which various distributions use to tag the release version.