Well, VoIP is just a very generic term describing the transport of voice over a IP-based connection, it doesn’t necessarily describe specific technology.
It tends to be used most often for SIP-transmissions, which would include VoWiFi in a broader sense.
Where VoWiFi differs is the tight integration in the modern carrier-grade telephony system called IP Multimedia Subsystem, aka IMS.
IMS has a lot of extra complexity, and there are basically 2 reasons for that:
- Integration into Carrier-Networks (including legacy support)
- Overall Quality
VoWiFi is expected to be very reliable, which includes a lot of roaming scenarios, it will allow better location functinality for emergency services (when you call 911, 999, or 112 or whatever your country is using), and it has a tightly integrated Quality-of-Service functionality.
The classic example would simply be support for roaming between mobile networks and your home WiFi, in which case a traditional SIP call would drop, Skype and others will either drop or at least severely lag, and in a proper VoWiFi situation neither party would notice.