Off-the-record messaging (also known as OTR) allows you you to have secure conversations with other people.
Using an account on Jabber and an appropriate messaging client (if you are using a PC, if you are using a Mac), you can encrypt your conversations once you have exchanged your public keys (a series of 40 digits and numbers) with the person you are speaking to. This way you can be assured that only the person you are speaking to will read what you are writing.
Always keep in mind, however, that you should not keep the logs of your conversations. If someone were to get hold of your device, this would defeat the purpose of the encryption. Check out the parameters of your client to insure that logging has been deactivated.
To find out how to set up OTR messaging, have a look at thedesigned by Security in-a-box.