Explain which of the following vector spaces are subspaces of eachother

${\cal C}(\mathbb{R})$

${\cal C}^n(\mathbb{R})$ (for different $n$)

${\cal C}^{\infty}(\mathbb{R})$.

I do not understand what ${\cal C}$, $n$ and $\mathbb{R}$ means in this context? Could someone clarify?