According to the definition as per Wikipedia®, Sildenafil citrate, sold under the names Viagra, Revatio and under various other names, is a drug used to treat male erectile dysfunction (impotence). (http://en.wikipedia.org/wiki/Viagra).

Sildenafil citrate is an oral medicine that is prescribed to treat impotence which is also called erectile dysfunction in males. This oral pill is known to work as well as its brand name. It works by increasing the flow of blood into the erectile organ, which helps in sustaining an erection which is required for satisfactory sexual activity.

Sildenafil citrate is most effective along with sexual stimulation. It is usually taken 30 minutes before sexual activity, with its effect lasting up to 4 hours. Generic Viagra works in the same manner as its brand equivalent, with its price being much less.   More detail...