To better understand the forthcoming discussion on probiotic strains, it is important to first be clear on how bacteria are named. Genus is the first name of a bacterium (e.g., Lactobacillus). It is somewhat general and refers to a grouping of organisms based on similarity of qualities, such as physical characteristics, metabolic end-products and metabolic requirements. Species is a bacterium’s second name (e.g., rhamnosus). It is a much more narrow classification based on shared common features that distinguish them from other species with that genus. Strain is an even more specific classification that distributes members of the same species into subgroups based on one or more properties that these bacteria have that are distinct from other members of the species (e.g., strain GG). 
An analogy that helps to conceptualize this bacterial classification system is that of dogs. Strains of bacteria within a given species can be likened to different breeds of dogs.All dogs belong to the genus Canis and the species familiaris. But as we know, within this one species there is great diversity in shape, size, strength, fur length, and other physical characteristics; ranging from the two extremes – the Irish wolfhound and the chihuahua. A similar division occurs within species of bacteria, where they will be a multitude of strains in any given species.
1. McKane, L. and J. Kandel, Microbiology: Essentials and Applications. 1986, New York: McGraw-Hill. 720-735.