I agree with answer of ACuriousMind, but I think it might also help to think about it like this....
$E^2=m_0^2c^4+p^2c^2 =m^2c^4$
where $m_0$ is the rest mass and $m$ is the relativistic mass.
The relatavistic mass increases as the momentum of the mass increases. At rest the two are equal to each other. As the speed of an object and its momentum increase the mass of the object increases.
so I think about it as
$E^2=m_0^2c^4+p^2c^2$
and
$E=mc^2$