Data are observations or facts which once collected, organised and evaluated, become information or knowledge. Data can take various forms, but is often numerical.
A data item is the smallest piece of information that can be obtained from a survey.
A variable is something measurable that is expected to either change over time or between individual observations. A variable can have any number of data items that relate to a specific measurement.
A data set is a collection of data items relating to a number of chosen variables.