The Virasoro algebra $\mathcal{V}$ is defined by the commutation relation

$$ [ L_n, L_m] = (n - m) L_{n + m} + \frac{c}{12} (n - 1)n(n + 1) \delta_{n, -m}$$

the number $c$ is called the central charge. 2d conformal field theories transform under two copies of the Virasoro algebra, $\mathcal{V} \, \times \, \bar{ \mathcal{V}} $. Chiral Conformal Field Theory transform under a single copy of the Virasoro algebra.

Conformal transformations are those which preserve angles. In two dimensions, this includes any holomorphic function $f(z)$. Infinitesimally, any such transformation takes the form

$$ z \to z + \epsilon z^{n + 1}$$

for integers $n$.

The action of these transformations on functions of $z$ is given by the differential operators

$$ \ell_n = - z^{n + 1} \frac{\partial}{\partial z} . $$

These operators satisfy the Witt algebra

$$[ \ell_n, \ell_m ] = (n - m) \ell_{m + n} . $$

$\ell_{-1}$, $\ell_{0}$, and $\ell_{1}$ generate a subalgebra which is often called the global part of the conformal algebra.

Now we would like to determine how conformal transformations act on the states in the theory. First, the algebra needs to be complexified, because the spectrum of a CFT is a complex vector space. This requires that we take complex linear combinations of the generators. The basis of the complex Witt algebra is denoted by $(\ell_n, \bar \ell_n)$. $\ell_n$ and $\bar \ell_n$ commute with each other, so this algebra acts on $C^2$ and we must take $z$ and $\bar z$ to be independent.

Second, we need to consider possible central extensions of the Witt algebra. This is because quantum states are only defined up to a total phase, so symmetry algebras need only act projectively on the states. A projective action of a symmetry alegebra is equivalent to the action of a centrally-extended algebra. The result is that the conformal transformations act on the states via the central extension of the Witt algebra, which is called the Virasoro algebra:

$$ [ L_n, L_m] = (n - m) L_{n + m} + \frac{c}{12} (n - 1)n(n + 1) \delta_{n, -m}.$$

Here $c$ is a generator which commutes with every other generator of the algebra, which means it is equal to a complex number times the identity. By abuse of notation, we also refer to this complex number as $c$– this is the central charge of the theory. It must be real for the theory to be unitary.

One of the axioms of 2d CFTs is that the spectrum breaks into irreps of the Virasoro algebra where $L_0$ is bounded from below. Consider such an irrep $\mathcal{R}$, whose lowest eigenvalue is $\Delta$, corresponding to an eigenstate $|\Delta\rangle$. It is easy to see from the structure of the algebra that $L_n$ decrease the $L_0$ eigenvalue for $n>0$. Therefore this state must satisfy

\begin{align} L_0 | \Delta \rangle \, &= \, \Delta | \Delta \rangle, \\ L_n | \Delta \rangle \, &= \, 0 , \qquad \quad n > 0. \end{align}

This allows us to define $\mathcal{R}$ by the $| \Delta \rangle$, plus every element of $\mathcal{V}$ applied to $| \Delta \rangle$. Schematically, these might include things like $ L_{-1}| \Delta \rangle$, $ L^2_{-1}| \Delta \rangle$, $ L_{-1} L_{-2} L_{-1}| \Delta \rangle$, $ L_{-1} L_{2} | \Delta \rangle$, etc. However we only need to consider $L_n$ with $n$ negative because the positive ones annihilate $| \Delta \rangle$. Furthermore, the commutation relations can be used to order the operators $L_n$ (meaning that unordered operators may be rewritten in terms of ordered ones). Therefore the representation consists of the states

\begin{align} & | \Delta \rangle, \\ & L_{-1} | \Delta \rangle, \\ & L_{-1}^2 | \Delta \rangle, \ \ L_{-2} | \Delta \rangle \\ & L_{-1}^3 | \Delta \rangle, \ \ L_{-1} L_{-2} | \Delta \rangle, \ \ L_{-3} | \Delta \rangle \\ &\ldots \end{align}

In this case, $\mathcal{R}$ is called a highest-weight representation. The collection $U(\mathcal{V})$ of all of the operators in the algebra is called the universal enveloping algebra of $\mathcal{V}$. As a result, we have $ R = U(\mathcal{V}) \, | \Delta \rangle $.

The lines in the above equation are organized by level, defined by

\begin{align} N = |L_{-n_1} \ldots L_{-n_p}| = \sum_{i = 1}^p n_i \, . \end{align}

Like most of the 2d CFT content, this page follows the excellent notes of Sylvain Ribault. The most complete form of those notes is given here