Statuspage.io lets you add a component for each of the separate infrastructure or functional part of your service. The best way to explain this is to show you a few of our customers who are using this feature correctly. Check out these Statuspages: Customer.io, New Relic, Vimeo, and KISSmetrics.
Unfortunately, the answer is "it depends". A good rule to use is to add a component for each major functional/architectural division of your service. Refer to the examples above as they do an awesome job splitting their service/product out into components that make sense to their customers.
To communicate the state of each component, they always have one of the 5 following statuses:
Component statuses are set when creating or updating incidents on your page.
Operational means exactly what it sounds like. The component is functioning as expected and in a timely manner.
Under Maintenance means exactly what it sounds like. The component is currently being worked on.
Degraded Performance means the component is working but is slow or otherwise impacted in a minor way. An example of this would be if you were experiencing an unusually high amount of traffic and the component was taking longer to perform its job than normal.
Components should be set to Partial Outage when they are completely broken for a subset of customers. An example of this would be if some subset of customer's data lived in a specific data center that was down. The component might be broken for that subset of customers but is working for the rest and thus there is a Partial Outage.
Components should be set to Major Outage when they are completely unavailable.
If you don't want to create an incident, but do want to update the status for a component, this can be done in the Components page. Find the component you wish to update, then click Edit. Here you can set the Status, then click Save Component to put that status into effect.