When fossil fuels burn, we mostly get three things: heat, water, and CO2. We also get some solid forms of carbon, like soot and grease.
So that's where all the old carbon goes. All that carbon stored in all those plants and animals over hundreds of millions of years is getting pumped back into the atmosphere over just one or two hundred years.
Did you know that burning 6.3 pounds of gasoline produces 20 pounds of carbon dioxide? Want to know how?