What is the difference between a "transition" and a "trigger"?
A transition creates a link between two states. It lets you define the condition when a record can move from one state to another.
For example, a customer record can move from "order placed" to "order confirmed" only if the "product is marked as available" by the stockist.
So here the transition is "product availability".
This transition can be triggered only if the stockists checks the availability and approves. So the approval of stock is the trigger.
Therefore, the record will move to the next state only when the transition happens which in turn will be triggered once the condition mentioned for the trigger is satisfied. If the stockist rejects the product availability then the transition will not take place, or the order will remain unconfirmed.