Dictionary.com defines statistics as the science that deals with the collection, classification, analysis, and interpretation of numerical facts or data, and that by use of mathematical theories of probability, imposes order and regularity on aggregates of more or less disparate elements.
A more understandable definition is simply the science of collecting, classifying, presenting, and interpreting numerical data.
Statistics is a practical discipline that changes with the needs of society. It is a useful tool for understanding the world we live in. It requires the use of mathematical formulas and concepts. It has applications in the fields of business, biology, engineering, industry, social sciences, commerce, and more. When you think about it, statistics is the universal language of the sciences. It allows us to accurately describe findings, make decisions, and make estimations. The field of statistics can theoretically be divided between descriptive statistics and inferential statistics.
Please see the eNotes link provided below for a more extensive review of statistics and its importance.
Just to tell you you should go to wolframalpha.com or dictionary.com for further definitions.