What is "clique-width"

In graph theory, the clique-width of a graph G is the minimum number of labels needed to construct G by means of the following 4 operations :

1. Creation of a new vertex v with label i ( noted i(v) )
2. Disjoint union of two labeled graphs G and H ( denoted G ⊕ H )
3. Joining by an edge every vertex labeled i to every vertex labeled j (denoted η(i,j)), where i ≠ j
4. Renaming label i to label j ( denoted ρ(i,j) )