Visit What is Digital Humanities to view definitions (you'll see a new definition every time you refresh the page)
A few definitions:
"Digital humanities is an area of scholarly activity at the intersection of computing or digital technologies and the disciplines of the humanities." (Wikipedia)
"Digital Humanities is a nexus of fields where scholars use computing technologies to investigate the kinds of questions that are traditional to the humanities, or, as is more true of my own work, ask traditional kinds of humanities-oriented questions about computing technologies.” (Kathleen Fitzpatrick, Director of Digital Humanities and Professor of English at Michigan State University)
See also, Chapter 1: "Defining Digital Humanities." In Debates in the Digital Humanities.
Another great way to learn more about digital humanities is to browse journals, and read about recent projects and research being done in the field.