Difference between SAP Ariba & SAP S/4 HANA Central Procurement.

Hi All,

I am confused and like to know...

Difference between SAP Ariba & SAP S/4 HANA Central Procurement. I want to invest time and money to learn one of this to make a career based on its merits.. checked some of the blogs & Q&A s but still not clear. Thanks and Regards