Let's start. Antibacterial agent(antibiotic) is classified broadly into bacteriostatic(which stops replication and bactericidal(which eventually kills the bacterial). As you know, bacteria has membrane, nuclear material, and different enzymes to do it purposes. So, the drugs act on one of this area. For example, penicillin acts on the cell wall and brings about disintegration by inhibiting the function of proteins(enzymes) on the wall. Other families, the cephalosporin, tetracyclines, microlides, bla bla. They got several drugs under them. When it comes to the antiviral. Virus doesn't most of the stuffs bacteria has but nuclear material(it is barely living organism). It needs enzyme to integrate it nuclear material to the human on. These enzymes are integrase(adds the viral DNA to human onel, reverse transcriptes(changes viral rna to DNA) , protease(sloughs off newly formed proteins to required products). So, the drug target one of this process. Eg. Zidovudine, acyclovir, atenavir... Their use depends on the indication, their side effects, their outcome, their price, patients preference, availability, the strain of the microrganisms(some organisms modify themselves in a way that helps them to evade drug action through mutation, epigenetic changes... So, in order to avoid such kind of resistance the drug type need to be changed)