Everything that happens in Shotgun creates a Shotgun event, and all the events are logged. You can see the events in your site by looking at the history tab on any entity page.
Note: You will need to add an "Event Log Entries" tab to the page in order to see the history of that entity.
The event log can be monitored. You can set up an event monitor that watches for events that fit certain criteria and when one occurs, it can trigger other actions. For example, you may want to watch for Animation Task statuses being switched from "In Progress" to "Complete," then automatically switch the Lighting Task from "Waiting to Start" to "Ready To Start."
You can also trigger events outside of Shotgun. For example, automate processing Versions in a Playlist to send to a client.
Because this is a pretty common thing for people to do, we have a standard version of an event monitor that you can download and customize called shotgunEvents.
Be aware that setting up the event monitor is an advanced use of Shotgun that is going to require a pipeline developer at the studio to write custom code. For more information, see "Writing event driven triggers.