Original: In the case of a block cipher, it refers to a set of techniques for tracing differences through the network of transformations, discovering where the cipher exhibits non-random behaviour, and exploiting such properties to recover the secret key.
Simplified: In the case of a block cipher, it refers to a set of techniques and methods for tracing differences through the network of transformations, discovering where the cipher demonstrates non-random behaviour, and extend such properties to recover the secret key.