Hello Jean-Christophe
Cisco Prime Infrastructure is a software package that that can be used to manage wired and wireless Cisco networks including:
…performance, application visibility, and user experience. It brings automation and best practices to the management of your Cisco® networks throughout the technology lifecycle, from design and deployment to day-to-day operations and service assurance.monitoring,
Cisco Prime Infrastrucutre can also be used to manage multiple instances of geographically separate CPI networks as one. More information about it can be found here.
APIC-EM on the other hand is something much more specific. It provides a method of implementing Software Defined Networking using traditional networking hardware.
I hope this has been helpful!
Laz