display all variables of currently logged in user:
env

display one variable value:
echo $SHELL

temporarily
create: VAR=test
show: echo $VAR
delete: unset VAR

permanent user variables:
~/.pam_environment and ~/.profile

After having edited one of those files, you should re-login (re-open terminal) in order to initialize the variables or use source command (run session script):
source ~/.profile

e.g.
PATH=${PATH}:/home/user/MyPrograms
...by default also /home/user/bin/ works

e.g. create new environment variable where multiple existing variables are combined:
nano ~/.profile
export C11="$HOME $USER"

which will set the variable and make it available to other programs.

If you want to set it for one invocation of a program, you can use:
HOMEUSER="$HOME $USER"

System-wide environment variables
---------------------------------
in Ubuntu:
/etc/environment

in some distros also /etc/profile could be used

https://help.ubuntu.com/community/EnvironmentVariables
http://askubuntu.com/questions/58814/how-do-i-add-environment-variables