It's easy to mix these up since compared to us, both are VERY SMALL. But bacteria, given the proper nutrients, can grow and reproduce on their own. Viruses cannot "live" or reproduce without getting inside some living cell, whether it's a plant, animal, or bacteria. And compared to viruses, bacteria are huge (see HowBig).
Coliphage T4 is a virus that looks like an alien landing pod. With its six legs, the bacteriophage attaches to the surface of the much larger bacteria Escherichia coli (E. coli). Once attached, the bacteriophage injects DNA into the bacterium. The DNA instructs the bacterium to produce masses of new viruses. So many are produced, that the E. coli bursts.