What's a Guard Node?
This is hop number one in the circuit; it's the first node you'll see when entering the Tor network. As such the guard node is the only node in the tor circuit that will see your origin IP Address.
What's an Internal Relay?
You could call this the middle hop; it comes after your passage through the guard relay. If you're just browsing the darknet; you will not enter the exit node, but only use the internal relays. These relays cannot see anything that you're doing; since each node is using layered encryption. Each time you enter a new relay; a new layer of encryption is formed.
What's an Exit Node?
This is the last hop in your circuit incase you're planning to enter the clearnet. In other words; if you decided to make connections to devices and sites outside the darknet. You will then be sent to this node where all the content will be decrypted to allow compatibility with the rest of the internet. Although you're protected against snooping if you're entering a site protected by SSL/TLS.