Antibiotics are selectively toxic compounds, typically small molecules, that can be administered to a human patient without severe side effects occurring, while killing bacteria. Antiseptics are toxic compounds preventing microbial growth, normally used to clean wounds. Antiseptics cannot be used internally, as they are too toxic. Disinfectants are toxic compounds capable of killing microorganisms on inanimate surfaces, such as fomites.
A good antibiotic will be capable of killing bacteria while leaving eukaryotic cells unharmed. This typically involves targeting a structure or enzyme that is exclusively found in prokaryotic cells, ensuring the molecule is selectively toxic and will not harm the patient.
The pharmacokinetics of a good antibiotic will include reaching the intended target to have a therapeutic effect, binding specifically to the target with few, if any, off-target effects, and being difficult to develop resistance to.
A good antibiotic will be challenging to develop resistance to, as the antibiotic becomes decidedly less useful when it doesn’t work against certain bacteria. This is difficult, and every clinically-used antibiotic discovered so far has had resistance arise in a few years after market release (except penicillin which had resistance demonstrated a number of years before its first clinical use).