The terms have become interchangeable, but are not exactly the same thing. Holy Hour is the “when” and Adoration is the “what”.
A Holy Hour before our Lord in the Blessed Sacrament is to remember, honor and even participate in the hour that Jesus asked Peter, and all of us by extension, to remain with him and stay awake at Gethsemane. The purpose of adoration of the Blessed Sacrament is a continuation of our worship of Christ in the Holy Eucharist, but outside of the mass.
Adoration is the first thing that our Lord deserves from us. He is deserving of our adoration simply because He is.
A couple of easy reads that increased my understanding of the Eucharist and adoration both at mass and outside of it are: 7 Secrets of the Eucharist by Vinny Flynn, and Celebrating the Holy Eucharist by Francis Cardinal Arinze.