The Chat Tab opens an chat messenger
Here is what the screen will look like with no messages in the chat window
Here is an example of the chat window with a message waiting to be read.
To Start a new message, click the red messages button in the lower right hand corner of the screen. This will bring you to your contacts page so you can text anyone in you contacts list.
You can also search for the specific name you want in the search bar like this. *blur out Will's number*
Once you are in the chat window of your choice, it pretty much like any other chat window out there today. Left side are the incoming messages, right side are the outgoing messages from you.