Whats the difference between watching movie online or downloading it?

When I watch movie online or when I download it then whats the difference in terms of bytes am using? Is it same? If yes, then why people watch movie online?
Whats the size of an average movie?

What is a VPN?

VPN is (Virtual Private Network) were first used by companies to enable their employees to securely access internal systems such as email remotely (e.g. from home or while on business trips). Today they are increasingly being used for personal use by individuals to protect their privacy while online in public places (e.g. when using the wi-fi connection in a cafe) or in a country where the internet is censored / blocked (e.g. China, Saudi Arabia��).