What is "distributed computing" as a field of computer science?
This is a TCS site, not a general computer science site, so I will only give the TCS answer.

In TCS, the following things are more or less equal:


"distributed computing"
"distributed algorithms"
the study of computational aspects of distributed systems (computability, computational complexity, algorithm design and analysis, etc.)
topics of conferences such as PODC and DISC
topics covered in textbooks such as Lynch (1996): Distributed Algorithms
topics covered in courses such as this.


(Outside TCS, "distributed computing" can mean anything: SETI@home, grids, clusters, clouds, HPC, Internet, web services, P2P, IPC, you name it.)