An instantiated DataType and all other data reachable from the DataStructure by following pointers and references. Many Algorithms are defined in relation to a DataStructure.

Show me your flowcharts and conceal your tables, and I shall continue to be mystified. Show me your tables, and I won't usually need your flowcharts; they'll be obvious.
— Frederick P. Brooks, Jr., The MythicalManMonth

Good DataStructure design is essential to writing clean, maintainable, efficient code.