“IP telephony is the Concept, VoIP is the Use”
I believe this quote can serves as an answer to the Big Question, but what does this really mean ?. To answer this question lets define each term separately.
IP telephony: It is a term used to describe the area of communication that is based on Internet standards like the Internet Protocol (IP). Examples of an IP telephony system include and IP Public Private Exchange (PBX). Please refer to this for more details on PBXs: http://en.wikipedia.org/wiki/Private_branch_exchange#Private_branch_exchange
Voice over Internet Protocol (VoIP): it is the set of standards that defines how Voice is digitized so that it can be transmitted across the internet. Examples include Session Initiation Protocol (SIP), H323, … and many others. For more details please refer to this link http://www.packetizer.com/ipmc/h323_vs_sip/
So Basically, IP telephony uses VoIP in order to be able to transfer digitized voice streams over the Internet:
If you are a programmer think of IP telephony as a Class and VoIP as a method in that class.
If you are not, simply think of IP telephony as the Car and VoIP as the type of the engine that car uses to travel from one place to another.
I hope this helps,
Have a good day =).