A Timer allows it to be executed automatically upon scheduling.
Several Timers can be executed at the same time, but the same Timer will never run simultaneously.
To know the meta-model and how Timers work, see this link.
A Process is executed based on the flow of activities to be performed at each stage of the entity's life cycle, as a task to be performed by the user, a Business Process.
See this link in the documentation and also this answer from @Leonardo Fernandes explains it very well.