Environmental Variables on APP Server and CI/DB

Dear All,

Can you all explain me in a very layman terms What does Environmental settings means. I am going to be administrating

R3/HP-UX/Oracle based system with Linux App servers. I need to know how can i edit environmental variables.

Do i use Vi editor or there are commands to set environmental variables. Please give me some knowledge and enlightenment on concept of environmental variables and how to set/edit them .

Appreciate all your support in anticipation.