A new era of Cloud Computing has emerged, but the characteristics of Cloud load in data centers are not perfectly clear. Yet this characterization is critical for the design of novel Cloud job and resource management systems. In this paper, the authors comprehensively characterize the job/task load and host load in a real-world production data center at Google Inc. They use a detailed trace of over 25 million tasks across over 12,500 hosts. They study the differences between a Google data center and other Grid/HPC systems, from the perspective of both work load (w.r.t. jobs and tasks) and host load (w.r.t. machines).