Difference between external activities and service activities

Hi All,

I am new to Project System module.

Please tell me the major difference between external activity and service activity with a suitable example.

I went through this thread "https://scn.sap.com/thread/1326524" but i didn't get any fair idea.All i know is external activity is activities related to vendors and service activities are using service management module.

Please clarify with a example if possible.

Regards,

shikha