Competitive inhibition is a type of enzyme inhibition which occurs when a substrate and an analog (similar structure as that of the subtrate) compete to bind with the enzyme. When analog concentration exceeds substrate concentration, the analog binds with the enzyme and the enzyme thereby gets blocked for the actual substrate to bind with it. As a result, the original (normal) enzymatic pathway is blocked and expected products are not formed.
Action of Sulfa drug is based on competitive inhibition.
In bacteria, folic acid is required for growth. It is sythesized from PABA (Para Amino Benzoid Acid).
Sulfa drugs (sulfonamides) compete with folic acid and bind with the enzyme (Dihydropteroate synthetase) which is responsible for conversion of PABA to Folic Acid. Hence folic acid synthesis gets blocked.