Rollout: A single-branch experiment, often the "winning branch" from an experiment, that can be used to "roll out" a feature to your Audience. See the Rollouts deep dive for more info.
What's the difference between Experimenter and Nimbus Nimbus is the whole experimentation system (handles enrolling/unenrolling users, sending branch configurations, etc). Nimbus was the code name of the experimentation project. If you hear Jetstream, that is the whole analysis system (interfaces with data, runs analysis, formats results). Experimenter is the customer interface to both Nimbus and Jetstream - that you access at experimenter.services.mozilla.com/. In Experimenter users can configure experiments, manage them (launch, monitor, end), and can see the results.