These notes are based on lectures delivered at the ‘Summer School on Quantization’ at Notre Dame University, May 31-June 4, 2011. Some additional material is included from a lecture series at the IGA workshop in Adelaide, September 2011. The audience for the summer school were postdoctoral and graduate students, with a variety of backgrounds. I made an effort to keep the lectures at a moderate pace, and to present motivation and foundational material, without going into technical details. These notes, while more detailed than the actual lectures, are written with a similar audience in mind. I thank David Li-Bland for his help in preparing this notes, and valuable comments.