A 50mg pill of Viagra contains 50 mg of active ingredient (Sildenafil concentrate) and a 100mg Viagra pill contains 100mg of Sildenafil.
A 50mg pill is the standard dose used to achieving the desired effect. It is important to take into consideration that the effect may vary for different people. One can begin with a 25mg pill (you can divide a 50mg pill in two pieces). Therefore one can decide if it is enough. One can take a 50mg pill if the desired effect is not reached.