VoIP or IP telephony is the technology allowing the convergence of data and voice networks, thus allowing the enterprise to save on maintenance, enhancing it's performance with more and better functions.
VoIP is the technology of the future, transforming voice signals to data ones, allowing the wide available networks such as internet to make calls for the mere cost of having an internet connection.
This is especially useful for enterprises with office branches, since the distance between each branch is rent meaningless, being able to make calls within the enterprise without any cost.
Asterisk is an open software application -under GPL license- of a telephonic switchboard (PBX). As any PBX, it can be connected to a determined number of telephones to allow internal calls, but it can be connected to VoIP provider or an RDSI line too. Asterisk includes many characteristics not found before on the cost-prohibitive, non open source PBX such as voice mail, long-distance calls, IVR, automatic call distribution and many more.
But the true potential of this switchboard resides on its versatility and adaptability to any given job: it is for example possible to connect it to your CRM so that whenever someone calls to your office, you get the caller's data displayed without any extra manipulation or that, just by calling an extension, to remotely switch on some lights.