The term data refers to information, particularly information that's stored in a uniform and systematic fashion. Inside a computer, each number, such as -97 or 31.125, is represented by a fixed number of 0's and 1's. Each character is assigned a numeric code. This code, of course, is stored in binary. Unicode is based on 16-bit codes, so the maximum number of unique characters is 65,536.
All programs use variables to hold pieces of data tempararily.
For example, if at some point in a program you ask a user for a number, you will store it in a variable so that you can use it later. The Celcius temperature Variables are given names by the programmer. We can choose whatever names we want, subject to the rules of the programming language. For example, you might declare one variable to have a type that allows it to hold numbers, and another variable to have a type so it can hold a person's name. The Java language requires the programmer to specifically define variables before they are used and state the type of value that will be stored in the variable.