StartActivity is how you would start an activity not associated with an interaction. StartActivityWithKey allows you to start an activity and have it be associated with an interaction. This comes in handy when you want to update a context during the activity as you can use the key provided at start to do update the correct one. In addition, closing an interaction can be initiated to cancel all running activities associated with the context.
A RobotActivity is what you would use when using the Pega Robotics Console in PRPC to communicate to the RPA process.