The first thing all mean the same thing. Which is using IP 
(Internet protocol) for voice services. Some voice networks 
are only packet-switched and have no access outside of 
their own VoIP network. Most VoIP networks have a Gateway 
that connects to a circuit-switched external network which 
gives them acces to external calling. One of the gateways 
responsibilites is to convert G.711 Circuit-switched media 
(typically a T1 provided by a telco company) to the 7.723 
Packet-switched media that will traverse the companies VoIP 
network. A device called a gatekeeper will then convert the 
IP address (used by H.323 protocol) to a standard telephone 
number (E.164 address) that can be used for external 
calling.  

A converged network is a network that passes both Voice and 
Data over the same set of devices. Converged networks 
generally implement QoS (Quality of service) on all actived 
network devices to ensure the VoIP has priority over 
standard data because of it's more rigid demands.