In chemistry a catalyst is a substance which, when added to other substances, allows them to change while itself remaining unchanged.
There are different catalysts, it depends on the chemical reaction. There is no one powder called catalyst.
a catalyst is a chemical used to speed up a reaction without being 'used up'
A biological catalyst is an enzyme, enzyme are found all over living things, each with specific use
In a regular fireplace you burn the fuel, wood, coal, gas or propane, and the byproducts of combustion go up the chimney or out the venting. In a catayltic fireplace those by products go into a catayst that burns at a much higher temperature so it further cumbusts much of the bi-product. The result is increased efficiency and less pollutants and combustion by-products.