gnu/linux uname and lsb_release
uname -a tells you which linux kernel you are using
$ uname -a
Linux myserver 2.6.30.10-105.2.23.fc11.i686.PAE #1 SMP Thu Feb 11 07:05:37 UTC 2010 i686 i686 i386 GNU/Linux
Linux myserver 3.8.0-22-generic #33~precise1-Ubuntu SMP Fri May 17 01:00:37 UTC 2013 i686 i686 i386 GNU/Linux
lsb_release tells you which gnu/linux distro you are using
$ lsb_release -a
LSB Version: :core-4.0-ia32:core-4.0-noarch:graphics-4.0-ia32:graphics-4.0-noarch:printing-4.0-ia32:printing-4.0-noarch
Distributor ID: Fedora
Description: Fedora release 11 (Leonidas)
Release: 11
Codename: Leonidas
No LSB modules are available.
Distributor ID: Ubuntu
Description: Ubuntu 12.04.2 LTS
Release: 12.04
Codename: precise