Serialization is the process by which data-structures are converted into a format that can be easily stored or transmitted and subsequently reconstructed.
Serialization is the process of converting a data-structures or object's state into a format that can be stored (for example, in a flat file or memory buffer) or transmitted over a network, to be reconstructed later maintaining data integrity - typically in a different run-time environment which may or may not be on the same computer.
The process of serializing an object is sometimes referred to as marshalling - however depending on the context / language this term may take on a slightly different meaning. The process of reconstructing a data structure from its serialized form is known as deserialization.