An ecig liquid usually has 3 ingredients - nicotine (to replace nicotine from cigarettes), flavouring (I'm vaping strawberry at the moment), and propylene glycol (the liquid). This is vaporised (no heat) and turned into a breathable vapour - similiar to an asthma inhaler.
From google - here is the info on cigarette ingredients.
'Cigarette smoke contains over 4,000 chemicals, including 43 known cancer-causing (carcinogenic) compounds and 400 other toxins. These cigarette ingredients include nicotine, tar, and carbon monoxide, as well as formaldehyde, ammonia, hydrogen cyanide, arsenic, and DDT. Nicotine is highly addictive.'
So in short you are going from 4,000 ingredients (some known carcinogens) to 3 (no known carcinogens). Yes zero would be better but this shows the immediate health benefit from switching. I can also get a lot of carcinogens for free (as you do) simply by breathing in traffic fumes when I leave the house - no need to smoke them.