KEYWORDS are the words whose meaning has already been explained to the compiler. Each specific keyword is used to implement each specific feature of the language. These keywords cannot be used as variable names or any other user defined element.
Example- The reserved words of C++ may be conveniently placed into several groups. In the first group we put those that are also present in the C programming language and have been carried over into C++. There are 32 of these, and here they are:

                                                      Table 1.1: Keywords of C and C++

There are another 30 reserved words that were not in C, are therefore new to C++, and they are:

Table 1.2: Keywords of C++