The rule engine allows you to define complex record rules and actions. Simply put, it's a process builder running in an Apex context.

• Allows easy creation of record-based rules using a simple plain UI.
• Can handle complex criteria and rules. Supports nested rules.
• Execute actions such as "Return a value" or "Call an apex method".
• Instant feature delivery of new changes.
• Supports rule translations in plain English for faster case resolution.

• Criteria - A criteria helps in evaluating a rule based on certain conditions.
• Single Rule - A single rule has criteria for evaluation and produces a result on successful evaluation.
• Nested Rule - A nested rule has criteria for evaluation but does not produce any result, rather it acts as a guard check for deeper single rules.
• Next Action - Next actions instruct the rule engine to stop or execute the next rule based on the rule configuration.
• Ruleset - Ruleset is grouping of multiple rules based on a single attribute/entry param. Like recordtype in salesforce.