Nicotine. Multiple chemicals are present but nicotine is the most important because of carcinogenesis and addiction reinforcing features via the reward system.
Too many to count. There are literally hundreds of organic compounds which, when burned, become the "tars" that stain your fingers, the filter (and your lungs). Cigarette smoke, in addition, to nicotine, also contains formaldehyde and carbon monoxide. Cigarette companies put in flavoring agents, menthol and other things, and actually the nature of these are considered trade secrets and they don't have to list them.