  Let $A$ be a bounded linear operator on a complex Hilbert space and $\Re(A)$
( $\Im(A)$ ) denote the real part (imaginary part) of A. Among other
refinements of the lower bounds for the numerical radius of $A$, we prove that
\begin{eqnarray*} w(A)&\geq &\frac{1}{2} \left \|A \right\| + \frac{ 1}{2} \mid
\|\Re(A)\|-\|\Im(A)\|\mid,\,\,\mbox{and}\\ w^2(A)&\geq& \frac{1}{4} \left
\|A^*A+AA^* \right\| + \frac{1}{2}\mid \|\Re(A)\|^2-\|\Im(A)\|^2 \mid,
\end{eqnarray*} where $w(A)$ is the numerical radius of the operator $A$. We
study the equality conditions for $w(A)=\frac{1}{2}\sqrt{\|A^*A+AA^*\|}$ and
prove that $w(A)=\frac{1}{2}\sqrt{\|A^*A+AA^*\|} $ if and only if the numerical
range of $A$ is a circular disk with center at the origin and radius
$\frac{1}{2}\sqrt{\|A^*A+AA^*\|} $.
  We also obtain upper bounds for the numerical radius of commutators of
operators which improve on the existing ones.
