Bad programmers worry about the code. Good programmers worry about data structures and their relationships.

What is Data Structures

a data structure is a data organization and storage format that is usually chosen for efficient access to data. -Wikipedia

This version explanation is a little academical, following is more precise.

a data structure is a collection of data values, the relationships among the values, and the functions or operations that can be applied to the data. -Wikipedia

So what is the mean of collection?

According to the Cambridge dictionary, its mean:a group of objects of one type that have been collected by one person or in one place.

To be simple, the data structure means that a number of data values, the relationships among them, and the functions or operations of one type of data that are put together by a data structure.

But there still exists some questions, such as type's meaning, value's meaning.

In this book, type is a basic tem whose meaning is determined by the precise definition of a type system. In any type system, types provide a division or classification of some universe of possible values:a type is a collection of values that share some property. Therefore, it always makes sense to ask what the elements of a type are. However, the kind of values that may have types,and the kinds of ditinctions we make in typing, may vary from system to system.
Throughtout this book, we will distinguish between types, which are collections, and values, which are members of types.
-Foundations for programming languages, Mitchell, 1996

So, we can illustrate the concept of data structure more insightful.

Before that, a value in computer science is the representation of some entity that can be manipulated by a program.

Now we can get a more compelete illustration about data structure showed in figure format.

What is Function ( in computer science )

a function / procedure / method / subroutine / routine / subprogram is a callable unit. -Wikipedia