strlen() is used to get the length of a string stored in an array.
sizeof() is used to get the actual size of any type of data in bytes.
Besides, sizeof() is a compile-time expression giving you the size of a type or a variable's type. It doesn't care about the value of the variable.
strlen() is a function that takes a pointer to a character, and walks the memory from this character on, looking for a null character. It counts the number of characters before it finds the null character. In other words, it gives you the length of a C-style null-terminated string.
The two are quite different. In C++, you do not need either very much, strlen() is for C-style strings, which should be replaced by C++-style std::strings, whereas the primary application for sizeof() in C is as an argument to functions like malloc(), memcpy() or memset(), all of which you shouldn't use in C++ (use new, std::copy(), and std::fill() or constructors).
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