Rules are the bases in which you will be able to configure automated actions to occur when specific events and conditions are met. The building blocks of rules are Events, Conditions and Actions.
Rules are triggered when an event is published that the rule is interested in, e.g. build synchronzied or build linked to issue. The rule is scheduled for execution as soon as an event is received that the rule was configured for.
Conditions are means to narrow the scope of a rule, e.g. only perform the rule actions when the build has a specific outcome, or when the build is of a specific job. Conditions can also be combines, e.g. only performs actions for successful builds of job X.
Actions are the building blocks of rules that will actually do something. They can create an issue, transiation an issue, comment on an issue.