‘Extend – An extend relationship from use case A to use case B indicates that an
instance of use case B may be augmented (subject to specific conditions specified in
the extension) by the behavior specified by A. The behavior is inserted at the
location defined by the extension point in B, which is referenced by the extend
relationship.
Include – An include relationship from use case E to use case F indicates that an
instance of the use case E will also contain the behavior as specified by F. The
behavior is included at the location which defined in E.’
quoted from OMG
通俗来说，建一座房子 include 建一个房门（因为每个房子都会有房门；安装通水管道 extend 建一座房子（因为并不是每个房子都要建通水管道）