A variable is used to hold data within your program. A variable represents a location in your computer's memory. You can put data into this location and retrieve data out of it. Every variable has two parts, a name and a data type.
Valid names can consist of letters, numbers and the underscore, but may not start with a number.
A variable name may not be a C keyword such as if, for, else, or while. Variable names are case sensitive. So, Age, AGE, aGE and AgE could be names for different variables, although this is not recommended since it would probably cause confusion and errors in your programs.
Is used to declare an integer variable. Note that integer variables truncate the remainder on division while floating point variables do not. On 32-bit machines, ints are typically stored in 32-bits and are equivalent to longs. On 64-bit machines, ints may be stored in 32 or 64 bits. Thus, they may be equivalent to either short or long. Check your system documentation.
Float variables are used to hold non-integer, floating point values.
Double variables are used to store floating-point values. They offer greater precision and can store larger numbers than floats.
keyword used to declare a character variable. Character variables are typically stored in one byte.
Bool variables are used for boolean, true/false values.
A declaration is used to specify the name and type of an object to the compiler. It asserts that the object exists, but does not actually allocate space for it or create it.
int myFunction(float arg1, int arg2 ); // Declaration of a function
extern float radius; //Declaration of a variable or object
class Cat; //Declaration of a class in C++