Each chunk contains a header which indicates some parameters (e.g. the type of chunk, comments, size etc.) In the middle there is a variable area containing data which are decoded by the program from the parameters in the header.
Chunks may also be fragments of information which are downloaded or managed by P2P programs.
In distributed computing, a chunk is a set of data which are sent to a processor or one of the parts of a computer for processing. For example a sub-set of rows of a matrix.