Defining a variable means allocating a chunk of RAM on the AVR and giving it a name. After a variable is defined, the chunk of memory can be assigned a value, and that value can be recalled, using its name. A variable definition looks like this:
u08 myvar; // unsigned 8-bit integer named myvar
where u08 is the type and "myvar" is the name. Notice the statement ends with a semicolon, like all C statements. A variable must be declared before it is used, and before any executable statements in the function in which it is declared. Integer types are:
u08 a; // unsigned 8-bit integer (0 to 255) or 0 to MAX_U08
s08 b; // signed 8-bit integer (-128 to 127) or MIN_S08 to MAX_S08
u16 c; // unsigned 16-bit integer (0 to 65535) or 0 to MAX_U16
s16 d; // signed 16-bit integer (-32768 to 32767) or MIN_S16 to MAX_S16
u32 e; // unsigned 32-bit integer (0 to 4294967295) or 0 to MAX_U32
s32 f; // signed 32-bit integer (-2147483648 to 2147483647) or MIN_S32 to MAX_S32
MAX_U08, etc. are macros that are #defined in the AVRLib.