Help & Training does say it all, but here's summaries:

* Escalation Rules: Causes a case to be escalated after a predefined time if it meets certain criteria. For example, if you have a SLA for 24 hour response time, you can have cases that fall outside this SLA be escalating, notifying a manager and flagging the case.

* Assignment Rules: Causes a lead or case to be automatically assigned to the correct individual or group when the record is created or modified. Common uses include lead assignment by sales region, and case assignment by specialist.

* Approval Process: Allows administrators to create a flow where records can be submitted for approval, and take action based on those approvals. Common examples include approval for a sales associate to offer more than certain dollar amount or percentage discount, or to make sure that accounts that are larger than a certain size can be handled by a certain sales representative. Submitting for approval sends the request to a specific person or a manager (or other chain of command), which can be approved or denied. Approvals may happen automatically when certain conditions exist.

* Time Based Workflow: SImilar to Escalation Rules, but they can work for any object that supports workflow rules. Common uses include opportunity expiration notices (e.g. some organizations require a sale be made within a certain timeframe or the sales associate loses the right to that sale), or automatically deactiving products on the day they are no longer going to be offered. There are many possible scenarios that can occur using this mechanism.