To find kernel version, run

uname -a

To find Linux distro version details, run

cat /etc/*release
cat /etc/*issue
cat /etc/*version

The actual command depends on what OS Distro you have. On Debian

root@hontest:~# cat /etc/*issue
Debian GNU/Linux 8 \n \l

root@hontest:~# 

On CloudLinux

root@server20 [~]# cat /etc/*release
CloudLinux release 7.1 (Vladimir Komarov) 
DISTRIB_ID=CentOS
DISTRIB_RELEASE=7
DISTRIB_CODENAME=
DISTRIB_DESCRIPTION=
NAME="CloudLinux"
VERSION="7.1 (Vladimir Komarov)"
ID="cloudlinux"
ID_LIKE="rhel fedora centos"
VERSION_ID="7.1"
PRETTY_NAME="CloudLinux 7.1 (Vladimir Komarov)"
ANSI_COLOR="0;31"
CPE_NAME="cpe:/o:cloudlinux:cloudlinux:7"
HOME_URL="https://www.cloudlinux.com/"
BUG_REPORT_URL="https://helpdesk.cloudlinux.com/"

CloudLinux release 7.1 (Vladimir Komarov) 
CloudLinux release 7.1 (Vladimir Komarov) 
root@server20 [~]# 

On Ununtu/Debian/Mint, you can install lsb-release with

apt-get install lsb-release

Then run, lsb-release -da

boby@fwhlin:~ $ lsb_release -da
No LSB modules are available.
Distributor ID:	Ubuntu
Description:	Ubuntu 14.04.3 LTS
Release:	14.04
Codename:	trusty
boby@fwhlin:~ $ 

For debian

root@hontest:~# lsb_release -da
No LSB modules are available.
Distributor ID:	Debian
Description:	Debian GNU/Linux 8.2 (jessie)
Release:	8.2
Codename:	jessie
root@hontest:~#