uname

uname -r

You should receive a result similar to the following:

2.6.32-431.11.2.el6.x86_64

The output can be interpreted with the following key:

2 – Kernel Version
6 – Major Revision
32 – Minor Revision
431.11.2.el6 – Fix/Revision Detail

Per the manual page, uname can also give the following information:

-a, --all                print all information
-s, --kernel-name        print the kernel name
-n, --nodename           print the network node hostname
-r, --kernel-release     print the kernel release
-v, --kernel-version     print the kernel version
-m, --machine            print the machine hardware name
-p, --processor          print the processor type or "unknown"
-i, --hardware-platform  print the hardware platform or "unknown"
-o, --operating-system   print the operating system
--help                   display this help and exit
--version                output version information and exit