1. sizeof is an operator, while strlen is a library function.
2. The parameter of sizeof could be data type or variable, while strlen can only have char* as parameter.
3. The sizeof is calculated during the compile process, while strlen is caculated during the executing time.
4. sizeof returns the size of the memory space the data occupies, while strlen returns the exact length of the string.
5. When use array as a parameter, sizeof return the size of the array in terms of byte, while strlen returns the length of the string(it is the same as to pass the pointer to strlen).