A grammar is said to be of Chomsky normal form if every production has either of the two forms $$ A \to BC \qquad \mbox{ or } \qquad A \to a $$ where $A,B,C$ are non-terminal symbols, and $a$ is a terminal symbol.

Grammars of this sort are context-free, hence they describe context-free languages. Moreover, given any context-free language not containing the empty word $\lambda$ , there exists a Chomsky normal form grammar which describes it.