  Cuntz algebra $\mathcal O_2$ is the universal $C^*$-algebra generated by two
isometries $s_1, s_2$ satisfying $s_1s_1^*+s_2s_2^*=1$. This is separable,
simple, infinite $C^*$-algebra containing a copy of any nuclear $C^*$-algebra.
The $C^*$-algebra $\mathcal O_2$ plays a central role in the modern theory of
$C^*$-algebras and appears in many substantial statements, including a
formulation of the celebrated Uniform Coefficient Theorem (UCT). There are
several extensions of this notion, including Cuntz algebra $\mathcal O_n$,
Cuntz-Krieger algebra $\mathcal O_A$ for a matrix $A$, Cuntz-Pimsner algebra
$\mathcal O_X$ and its relaxation by Katsura for a $C^*$-correspondence $X$,
and Cuntz-Nica-Pimsner algebra $\mathcal {NO}_X$, for a product system $X$. We
give an overview of the construction of these classes of $C^*$-algebras with a
focus on conditions ensuring their simplicity, which is needed in the Elliott
Classification Program, as it erature, except our discussion on the sufficient
conditions for simplicity of the reduced Cuntz-Nica-Pimsner algebra
$\mathcal{NO}^r_X$, which is known to expertsstands now. The results we present
are now part of the lit, but might happen to be new for some of our audiences.
