Session Initiation Protocol represents a set of protocols that facilitate a session running over a data network. A SIP session could be voice-based, in which case, it’s analogous to making phone calls over the PSTN.
SIP can support other media modes, such as video and instant messaging. But VoIP is only about voice, so SIP will be one of the protocols used to enable calls over a data network. Just to clarify: Both acronyms have the word protocol, but only SIP is a protocol. With VoIP, voice calls are made over the internet, so it’s not a protocol itself. Rather, VoIP is a set of voice technologies that support internet protocols, which enables calls to traverse a data network.