If you don't want Google to store your chat history and the Google chat history of the person you are chatting with, you can take the chat off the record and this is how you do that.
You got a .rar file and you don't how to open it, some one asked you to send him "a rar file" or you just need a good archiving software? Download and install WinRAR.
If you are using Facebook social apps for watching videos, reading content and you don't want your friends or everyone to know about your activity, you can turn this off.