傻强学用例

A scenario is a sequence of steps describing an interaction between a user and a system.
场景是描述用户和系统间交互的若干步骤的序列。

A use case is a set of scenarios tied together by a common user goal.
用例是一组拥有共同用户目标的场景的集合。

Actor isn't really the right term; role would be much better. Apparently, there was a mistranslation from Swedish, and actor is the term the use case community uses.
“活动者”不是个正确的术语，“角色”要更好一些。显然，这是一个自瑞典语的翻译错误，并且“活动者”也是用例社群使用的术语。