The following questions are answered here: How to share your Google Drive with your friends
How to upload files and folders to Google Drive
How to send a link to your friend so that he can download your files and folders
How to download files from the share link your friend has sent to you
What is Google Drive? How it is useful to me?
You can keep very important files in the drive which is online. 5GB space is free.
So keep a copy of some of your important files in this google drive. So that even if your harddisk crashes, you need not worry.
If you want to send certain fails to your friend, normally you will be using your email eg. gmail.
But in case of gmail, the maximum file size is 25mb. In google drive, you can send even 10 gb.
See the following screenshots to know how it works.