Here's a breakdown:

* Decomposition reaction: A single reactant breaks down into two or more products.

* Example: 2H₂O → 2H₂ + O₂ (Water decomposes into hydrogen and oxygen gas)

* Combination reaction (or synthesis reaction): Two or more reactants combine to form a single product.

* Example: 2H₂ + O₂ → 2H₂O (Hydrogen and oxygen gas combine to form water)

You can see that the decomposition reaction and combination reaction are essentially reverse processes of each other.