Here is quick definition:

http://en.wikipedia.org/wiki/Summons

I am not sure about Maryland, but in Missouri if they are trying to serve a civil summons it would be considered a subpoena, order to appear, but if it is a criminal summons, possession, it is most likely a warrant already and they are just describing it nicely.  If the Sheriff is having problems locating the person then they probably already know they have a warrant and are trying to lay low.

Jay