What is DATA?
Data is a plural of datum, which is originally a Latin noun meaning "something given." Today, data is used in English both as a plural noun meaning "facts or pieces of information" and as a singular mass noun meaning "information".

Information refers to data being arranged and presented in a systematic or an organize form, so that some useful inferences can be drawn from the same. By data we generally mean quantities, figures, statistics, relating to any event.

What is DATA INTERPRETATION?
Data Interpretation is the extraction of maximum information from a given set of data or information. The data can be presented either in the form of a table or a bar chart or a pie chart or a line graph or as a combination of one of these formats.

In CAT there is a separate section for DI & DS. In past history DI section used to consist of 5-6 sets of data followed by 5-6 easy but lengthy calculation based questions on each set. But the trend of DI section has changed a lot in last couple of years now the questions are of more logical requiring less calculation.

In the beginning one should try to get familiarize with all data formats and solve easy questions based on single data/graph. But In real CAT questions are usually asked on combinational graph.
Before starting DI section one should be very comfortable with numbers, calculations, fractions and percentages. It helps in reducing the time required for solving the questions.

To begin with, you should know how to interpret various data formats. Tables, line graphs, bar graphs, pie charts, three dimensional graphs, flow diagrams and cumulative frequency tables are some of the data formats that have featured in CAT over the years.

Before moving to different chapters read the following 5 tips on DI @ http://www.cat4mba.com/node/150
Go through all the chapters carefully. Solve the questions based on simple logic. After completing all the chapters try to solve the questions in question bank and then try all the FLTs.