said by Glen1 :

A demarcation point is where the telephone company equipment stops and the customer's equipment starts. Over the years there have been many definitions put forth. Bell was going to install a demarc for each and every customer but found the cost would be prohibitive. Some people believe the demarc to be the telephone "protector" in the basement but this is not really a true demarc point as the customer cannot really do what is supposed to be done. At a proper demarc the customer should be able to unplug the inside wiring and plug in a single telephone to see whether the trouble is on the customer's wiring or beyond. The ideal demarc would be a single jack just past the "protector" (lightning protector) where the customer could unplug his inside wiring and plug in a single telephone to see if trouble still existed.

said by mau108 :

I have a phone jack that my wiring person installed which connects the wire comming from outside (bells demarc) to my wiring, I guess that can be used as a demarc if needed...its right my electrical box...

said by bylo :

Demarcs come in many flavours depending on the type of home and the age of the installation. In addition to the external box illustrated in the link upthread there are several varieties that are installed inside the house. In my townhouse, the demarc is just a small pushdown block. In older apartments there may be no official demarc into the unit. In newer ones there could be a jack on the wall near the electrical panel. And so on...