Probiotic means "agent for life" as against Antibiotic which means "against life".
Accordingly, the use of Antibiotic will kill all living organisms in your system whether friendly or unfriendly. Our digestive system esepcially needs friendly bacterium to help digest and break down food for better absorption.
Probiotic is more particularly applied to lactic acid bacterium. These are very natural to our digestive system and is in mother's milk - lactobacilli and bifidobacterium. Lactic bacterium produces natural Antibiotics which are anti-septic to harmful and pathogenic bacterium. They are therefore able to help the digestive system to eliminate and clean bad bacterium and toxins.
There are countless strains of lactic bacterium but not all helpful or beneficial.