2 million stolen passwords from various websites.

Hello. Recently, I heard of an issue going around that 2 million passwords were stolen from various websites. There's an article here from NBC news.

 

http://www.nbcnews.com/technology/2-million-stolen-passwords-facebook-twitter-google-yahoo-others-leaked-2D11691630

 

I don't really fully understand the situation. Is there anything that we can do to determine if we have a problem or how to prevent one? Thanks!