VoIP is 'Voice over the Internet Protocol' - the Layer 3 protocol that defines how voice travels over a LAN/WAN network.
IPT stands for IP Telefony. IP telephony transmits voice communications over the network using open, standards-based Internet Protocol.
Today these systems are complete voice-system , including callcontrol software and end-devices such as phones. Also these systems have open API's to interconnect with third party applications and support video to make a complete Unified Communication
These days, both of these terms are often used interchangeably. However, they are technically different. Neither is vendor-specific - in other words, saying VoIP or IPT does not inherently mean "Cisco" or "Avaya" or "", etc. Here are the major differences:
VoIP is a protocol. Specifically, it just defines a means for voice to travel over IP vs. the traditional TDM/POTS methods. When you say, we use "VoIP" you are simply saying that you are making phone calls over IP on a converged network (typically) that prioritizes (recommended) voice and data traffic.
IP Telephony or IPT is more of an infrastructure-related term. Your IPT infrastructure would include your IP-PBX, gateways, IP phones, and etc. Your IPT infrastructure could be any vendor but it is the backbone or the core of what you are using to fully implement a VoIP environment.