To enter into a computer the information required to begin a session.
进入，注册，登录：往计算机内输入开始一个程序的信息
To enter into a computer the command to end a session.
退出，注销：往计算机内输入结束一个程序的信息
(informal)very easy
(非正式)易如反掌
go through the procedures to begin use of a computer system, which includes establishing the identity of the user
进入（系统），注册，登录
go through the procedures to conclude use of a computer system
退出（系统），注销