The foremost difference between MPX and IP cameras can be found in the way that they compress video data.
MPX cameras capture the raw video data, convert it to digital for processing, then convert it back to analog to send it over BNC coaxial cables to the DVR. The DVR will then compress the data using H.264 advanced video coding before storing it to the hard drive.
Digital IP cameras, on the other hand, are more like little computers themselves. They will first convert the original analog video data to digital for processing. They will then encode the footage themselves (using H.264 or H.265 encoding) and ultimately send the data over Cat5e Ethernet cables back to the NVR for storage.