It is a device capturing and controling all the signals, which are being created by the whole system, especially the made and received phone call signals. The device is mostly used for connecting regular phone system with the Internet and backwards.
Obviously SIP is considerably better than H.323 and complies with the understanding of IP-telephony as a global mass IP-service, and it should be implicitly considered right this way. And as SIP is able to serve not only telephony but also any communications at all in the real time over the IP-protocol; obviously it should become a base for other IP-services not yet existing but will appear in the future, as well.