Answer:
If you are referring to the smoke released by a fire, they contain tiny particals that reflect light and also they contain Carbon Dioxide and other gases that some may be toxic (such as Carbon Monoxide).
If you are referring to cigarettes, they contain the leaf of the tobacco plant along with hundreds of chemicals, some highly addictive, such as nicotine, which is very addictive and is a supertoxin. They also contain tar and other chemicals that can increase the chance of cancer.