A rule has a description and associated match conditions and actions. These specify, when a rule is to be carried out. Again, there can be no, one or many criterias for a single rule. If there is no criteria, the rule will always match. This is useful in many cases.
Actions carry out the actual processing of a messages.
For example, actions include logging a message to a flat file or database, sending it via email or forwarding it to another syslog daemon. There can be no, one or an unlimited number of actions associated with a rule. However, if no action is associated, the rule will not have any effect. Consequently, the client will issue a warning when writing the rule base. Rules without actions can be useful to temporarily disable a rule with complex criterias.