Viagra and Sildenafil, both have the same active ingredient (Sildenafil) and are equally effective.
Sildenafil is the generic unbranded version of Viagra. Viagra was the brand name used by Pfizer: the first company to license the treatment Sildenafil (a process which costs pharmaceutical companies a significant amount).
Once the Viagra patent expired, other manufacturers could make it at a lower cost. This is a good thing for you, as now you can get a cheaper medication which is just as effective.