Message is the built-in chat client included with OS X. You can use it to send messages using iMessage, or one of many third party messaging services.

Setting up accounts
If you have an Apple ID, you already have an iMessage account you can use to send and receive messages. Your Apple ID is usually the same account name and password you use with iCloud, the iTunes Store, or Mac App Store.

If you haven't already set up your Apple ID on your Mac, you're prompted to enter it the first time you open the Messages app.