If you're using Google Hangouts as your main messaging service, you might want to know that Hangouts doesn't use end-to-end encryption (E2EE), a must-have feature for messaging services in the post-Snowden world.
As far as messaging services go, end-to-end encryption is a method of encrypting data so that only the sender and the recipient of a certain message can make sense of the data being transferred. The main thing to bear in mind is that the provider of an E2EE-encrypted messaging service cannot view the messages itself, as the data is encrypted and decrypted locally by the sender and the recipient.
While the service provider has access to the bits of information that are transmitted between the sender and the recipient, this data looks like complete gibberish without the encryption key. It's worth noting that Whatsapp, the largest messaging service in the world, uses end-to-end encryption, as does Apple's iMessage.