Slash commands are typed in the console. These always start with a forward slash, /like /this.

Ghost chat is a really nice feature that reduces clutter and adds a little immersion, although it is not as useful if you like to chat a lot! To enable ghost chat simply click the little red cross. That is click the window close button instead of using the chat toggle key. From there on, the chat will pop up when someone talks, and will disappear after a few seconds. You can configure how long the ghost-chat should last in Options > GUI and Windows, along with the transparency level.