To quote the classic and famous Gardner's Art Through the Ages,

"Except when referring to the modern academic discipline, people do not often juxtapose the words 'art' and 'history.' They tend to think of history as the record and interpretation of past human actions, particularly social and political actions. Most think of art, quite correctly, as part of the present - as something people can see and touch. Of course, people cannot see or touch history's vanished human events, but a visible, tangible artwork is a kind of persisting event. One or more artists made it at a certain time and in a specific place, even if no one today just knows who, when, where, or why. ... Art historians seek to achieve a full understanding not only of why these 'persisting events' of human history look the way they do, but also of why the artistic events happened at all."

The concept of art history is expanded upon in an article on Caravaggista. A list of recommended reading for those interested in learning more about the fundamentals of art history can be found here. Eager readers may want search Google Books for 'art history,' or read previews of commonly used art history books here (art historical methods), here (Gardner's Art Through the Ages), and here (a primary source, Vasari's Lives of the Artists).

Excellent question, and the second most frequent question I receive! Because of that, I wrote this post, Why Choose Art History?,  for current art history students and prospective art history students alike to be able to get a taste of what they can expect to do with their degree (undergraduate or advanced). You can also read more Q&A from current art history students here (from a student who would prefer not to commence graduate study) and here (from a student wondering if she should enroll in a dual-M.A. program).

Andrew Graham-Dixon, A Life Sacred and Profane
Howard Hibbard, Caravaggio
Helen Langdon, Caravaggio: A Life
Francine Prose, Caravaggio: Painter of Miracles
Catherine Puglisi, Caravaggio
Peter Robb, M: The Man Who Became Caravaggio (this book is controversial)
Keith Sciberras & David Stone,  Caravaggio and the Knights of Malta
John Varriano, Caravaggio: The Art of Realism
Genevieve Warwick (ed.), Caravaggio: realism, rebellion, reception

The following are journal articles or part of anthologies, but they are (to me) as insightful as any book.

Joseph F. Chorpenning, Another Look at Caravaggio & Religion (Artibus et Historiae)
Philip Sohm, Caravaggio's Deaths (Art Bulletin)
John Varriano, Caravaggio & Violence
John Varriano, Caravaggio & Religion

Much more about Caravaggio exists in print. Caravaggio.com has a wonderful and extensive list of scholars have worked on Caravaggio, along with citations of their respective works.

Properly, the Caravaggisti were a group of Caravaggio-esque painters from the 16th and 17th centuries. That is, these artists painted in or were influenced by Caravaggio's brooding, dramatic, and realistic (unidealizing) style. I personally don't maintain the belief that after the 17th century, artists ceased to be Caravaggisti; the time "constraints" exist to delineate a a group of artists relatively close to Caravaggio's lifetime who could have been more easily or absolutely were influenced by Caravaggio directly.

For more information about the Caravaggisti, I recommend the following books (although this of course is not an exhaustive list):