Data structure is a part of IT theory and I guess can not be explained in few words. However, the simplest answer is that data structure tell us in what way the data is organized. One can have simple data structures like lists, sets, touples or more complicated ones being combination of ones mention above.

If you think about form of data I would say think about row data or organized data.

If you believe you need more detailed answer, than please provide us with the information why you need this answer?

From a very different perspective to the previous 2 answers - this is mine from a statistical modelling perspective:

On structures

http://www.bristol.ac.uk/cmm/learning/multilevel-models/data-structures.html

and on form (of the response variable)

http://www.bristol.ac.uk/cmm/learning/multilevel-models/types.html

And much more discussion in the free Lemma course

http://www.bristol.ac.uk/cmm/learning/online-course/index.html

One aspect that is important is also the purpose of use. I am mostly involved in health care data structures. These have generic formats, e.g. the conceptual level, which is the concept found in the real world and how that is represented through description or definition. e.g. ebola. What is ebola? How is it defined? What are characteristics. (e.g. symptoms / signs / cause).

Next level of representation is a logical model. I personally use Unified Modeling Language (UML) class diagrams, and use specific approach that each single data element must become one class. Reason for this is that on level 3, implementation, it needs to be useful. Others would use OWL for it.

The implementation level meets the technical world more, and examples in the answers above are included, such as RDF, CSV, XML, JSON, ADL and more. The clinical relevant thing is that anything specified at conceptual level must medically be correct, the logical (UML) model must specify the full data elements, their relationships and their characteristics. One characteristic is for instance the nature of data type: text, numerical, ordinal, quantity, coded answer values, picture, time stamp or such.

I would suggest to be more precise on your goal first. Then look at the level you want to answer the question (conceptual, so in a specific domain), logical (e.g. being independent of any technology), or implementation specific (how would it look in XML or JSON etc.).

Article Detailed Clinical Models: Representing Knowledge, Data and S...

Dear @Krystian Wojtkiewicz, can you suggest me any book or any material regarding this problem.

Dear @Dhruba Das there are numerous books regarding this topic, however the question is, from which perspective you would like to study it. Below I tried to provide you with basic material, which can be later studied more by you in a direction needed by your research:

Very nice online course: http://www.cs.umd.edu/class/spring2008/cmsc420/

You will find there also some bibliography links to the books in the area.

Little introduction to programming: http://videolectures.net/mit600f08_intro_computer_science_programming/ 

which can be very interesting and surely will show you the IT "data" perspective 

If you need any more material, or more sophisticated one, please tell why you need it, so I could try to find something more suitable.

Dear @Krystian Wojtkiewicz, Actually it is a home assignment for me on research methodology in my Phd Course work. I only want to know about what is the form of data and their structures, I have never read that.

Dear Krystian,

What is the domain in which you do your PhD, it is quite a challenge to properly answer you if you do not give more details. It might be a difference in how physics, sociology and medicine look at data structures. Or is is basic IT as your reply to Dhruba might suggest?