1, variable: it is a memory area, and a memory contains two attributes, one is the address and the other is the stored content, that is, the value. So in C language, each variable has an address and its own stored content.

2. Constant: refers to the quantity that will not be modified. This concept actually comes from the concept of immediate number in assembly language and is stored in instruction code.

3. Integer: used to describe the data type of integer.

4. Long integers, short integers and unsigned integers are mainly distinguished by the sign of integers and the size of bytes occupied by memory. As an integer, it can be divided into positive and negative numbers, so there is a difference between unsigned number and signed number in C language. The minimum value of an unsigned integer is 0 and the maximum value is 2 n- 1, where n is the number of bytes occupied by the type in memory. Generally speaking, int type accounts for 4 bytes, short integer accounts for 2 bytes and long integer accounts for 4 or 8 bytes. The specific byte size can be obtained by the sizeof operator.