The chemical reactions that students meet first usually go to completion in one direction and it's clear what the reactants and products are.

A good example is the reaction of magnesium with hydrochloric acid:

Mg(s) + 2HCl(aq) ↓  MgCl₂(aq) + H₂(g)

Magnesium and hydrochloric acid are reactants and magnesium chloride and hydrogen are products.

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This reaction cannot be reversed. Bubbling hydrogen through a solution of magnesium chloride doesn't produce magnesium and hydrochloric acid.

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In some cases, however, reactants can be changed into products and the products can be changed back into reactants. These reactions are reversible.

For example, when solid ammonium chloride is heated in a test tube it decomposes into the gases ammonia and hydrogen chloride.

NH₄Cl(s) ↓  NH₃(g) + HCl(g) 

The gases react higher up the tube where it is cooler to form solid ammonium chloride.

NH₃(g) + HCl(g) ↓  NH₄Cl(s)

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How do you decide whether to call ammonium chloride a reactant or a product?

It can be either – it depends what you start with. If you heat ammonium chloride it's the reactant. If you start with ammonia and hydrogen chloride and make ammonium chloride it's the product.