Viagra is a cultural phenomenon by now, and there is not a man in the world that doesn’t know of that name, it’s a solution to every man’s worst nightmare, it makes men stay potent for much longer in the bedroom and helps with achieving erection itself. As to how does this cure achieve that, it’s pretty simple.
The idea behind Viagra is not to tense up or firm up the muscles of male reproductive organ, no, it works in a bit roundabout way. The erection itself is not a thing of the muscles or bones, it’s a simple physical process involving pressure – when a man is excited the blood flow to penis is increased and it goes hard due to the extra pressure. The process behind this is interesting, as it all starts with the brain, when it percieves something a man considers sexually exciting, nitric oxide starts getting produced and it is distributed through the body via blood. It induces an enzyme that gives a chemical named cGMP as it’s end result.
That chemical regulates flow of blood where it matters, it makes the blood valve that brings the blood into penis slightly larger and it slightly reduces the outflow valve, letting a man achieve erection. At the same time a chemical called PDE is produced, and this one can be called a “mood killer” – it inhibits cGMP. Viagra butts in at this stage of the process, bonding to PDE and not letting it inhibit cGMP, giving man much longer uptime so to speak.
Now, like any other medicine or a drug, Viagra has it’s quirks and downsides, the major one being the fact that the chemicals regulating blood flow 
in and out of penis are very similar to some of the chemicals found in the brain. While scientists managed to make Viagra a very targeted drug, there is some spill over effect, that’s why most men see slight blue sheen over everything some time after taking Viagra – the parts of the brain dedicated to vision are slightly affected. Also, people with heart symptoms are advised to stay away from Viagra as it can lead to complications. Alcohol does not present problems for Viagra, but if you’re on any kind of drugs that regulate nitric oxide, you should stay away from it, so consult doctor before use.
Naturally, Viagra is only for men, although there have been rumors of female version of the drug being in research, nothing has been confirmed quite yet. The reason behind difficulties of creating a Viagra for women is the fact that it is not based on chemical processes and on physiology, it’s more about lack of desire for sex, something that is very hard to cure by chemicals.