The term antibiotic (from Greek αντί - anti, "against" + βιοτικός - biotikos, "fit for life") was coined by Selman Waksman in 1942 to describe any substance produced by a micro-organism that is antagonistic to the growth of other micro-organisms in high dilution. This definition therefore excludes naturally occurring substances, such as gastric juice and hydrogen peroxide, which may kill micro-organisms but are not produced by micro-organisms.

Antibiotics are usually used to treat infections caused by bacteria. They do not work against other organisms such as fungi or infectious agents such as viruses . It's important to bear this in mind if you think you have some sort of infection, because many common illnesses, particularly of the upper respiratory tract such as the common cold and sore throats, are usually caused by viruses. Overuse of antibiotics can lead to bacteria becoming resistant to them so it's important to only take them when necessary.

The main class of Antibiotics are:
Macrolides
There are a couple of new relatives of erythromycin (azithromycin and clarithromycin) that work the same way, but kill more bugs and have slightly fewer side effects. The erythromycin-like antibiotics are also known as macrolides. Macrolides belong to the polyketide class of natural products. Macrolide antibiotics are used to treat respiratory tract infections, genital, gastrointestinal tract, soft tissue infections caused by susceptible strains of specific bacteria.
The most commonly-prescribed macrolides:

• erythromycin
• clarithromycin
• azithromycin
• roxithromycin

Aminoglycosides
Aminoglycoside antibiotics are used to treat infections caused by gram-negative bacteria. Aminoglycosides may be used along with penicillins or cephalosporins to give a two-pronged attack on the bacteria. Aminoglycosides work quite well, but bacteria can become resistant to them. Since aminoglycosides are broken down easily in the stomach, they can't be given by mouth and must be injected. When injected, their side effects include possible damage to the ears and to the kidneys. This can be minimized by checking the amount of the drug in the blood and adjusting the dose so that there is enough drug to kill bacteria but not too much of it. Generally, aminoglycosides are given for short time periods.
The most commonly-prescribed aminoglycosides:

• amikacin
• gentamicin
• kanamycin
• neomycin
• streptomycin
• tobramycin

Cephalosporins
Cephalosporins are grouped into "generations" by their antimicrobial properties. Cephalosporins are categorized chronically, and are therefore divided into first, second, third and fourth generations. Cephalosporins are used to treat pneumonia, strep throat, staph infections, tonsillitis, bronchitis, otitis media, various types of skin infections, gonorrhea. Cephalosporin antibiotics are also commonly used for surgical prophylaxis. Cephalosporins are closely related to the penicillins.
The most commonly-prescribed cephalosporins:
First generation

• cephazolin
• cefadroxil
• cephalexin
• cephradine

Second generation

• cefaclor
• cefuroxime
• cefprozil
• loracarbef

Third generation

• cefotaxime
• cefixime
• cefpodoxime
• ceftazidime
• cefdinir

Fourth generation

• cefepime
• cefpirome

Penicillins
Penicillin was the first antibiotic discovered by Alexander Fleming in 1929. Penicillins are used to treat skin infections, dental infections, ear infections, respiratory tract infections, urinary tract infections, gonorrhea. Penicillins are sometimes combined with other ingredients called beta-lactamase inhibitors, which protect the penicillin from bacterial enzymes that may destroy it before it can do its work. Penicillins are usually very safe. The greatest risk is an allergic reaction, which can be severe. People who have been allergic to cephalosporins are likely to be allergic to penicillins.
The most commonly-prescribed penicillins:

• amoxicillin
• ampicillin
• bacampicillin
• oxacillin
• penicillin

Tetracyclines
Tetracyclines are a family of antibiotics used to treat a broad spectrum of bacterial infections. Tetracyclines were discovered in the late 1940s and were extremely popular when they were first discovered. The tetracycline antibiotics have a very broad spectrum of action. Tetracyclines are used to treat mild acne, Rocky Mountain spotted fever, Lyme Disease, upper respiratory tract infections, urinary tract infections, sexually transmitted diseases, typhus.
The most commonly-prescribed tetracyclines:

• tetracycline
• doxycycline
• minocycline

An important thing to remember is if you take antibiotics when you do not need them, they may not work when you do need them. Each time you take antibiotics, you are more likely to have some bacteria that the medicine does not kill. Over time these bacteria change (mutate) and become harder to kill. The antibiotics that used to kill them no longer work. These bacteria are called antibiotic-resistant bacteria. These tougher bacteria can cause longer and more serious infections. To treat them you may need different, stronger antibiotics that cost more. A stronger antibiotic may have more side effects than the first medicine.
Some of the side effect that antibiotic can caused are:

• Nausea.
• Diarrhea.
• Stomach pain