Difference between C & C++

Both STR and arr allocate a char array in a global memory area or on the stack containing the characters on the stack. The differences:
1. arr: arr is an identifier whose value is a constant, there is no variable allocated for the value of arr itself. When you use arr in your code then the compiler "hardcodes" this constant. (Under the hood it isn't necessarily a constant when the machine code is generated for offset independent stuff and when the location of the array is a stack relative memory area but this is the problem of the code generator, from your perspective (from the perspective of the language user) arr is a constant value without allocated variable/storage.)
- STR: In case of this declaration besides the character array the compiler also allocates a char* variable for you and initializes it for you to the address of the character array (to the constant value that is the same value you access with the arr symbol in the previous example). A very big difference compared to arr is that STR here itself is a pointer VARIABLE that you can modify later in your code, you can not do that with the arr CONSTANT.

Another very big difference: the type of the STR and arr symbols. The type of STR is char* while the type of arr is char[]. The difference is often invisible because the C/C++ compilers contain a very ugly *hack* (in my opinion): array identifiers (arr) are automatically casted to pointers in many cases. For example if you say char* myvariable = arr; then the compiler automatically casts the type of arr into a pointer that points to arr.

sizeof also has surprises: sizeof(STR) is the same as sizeof(void*) as STR is a pointer VARIABLE and sizeof(arr) will give you the size of the character array as arr is a character array. However if you declare a function like:
void myfunc(char array_param[])
{
}
then sizeof(array_param) is sizeof(void*) because array parameters are pointers in reality, array parameters are never passed by value even if you specify the exact size of the array parameter!!! You can enforce a fixed size for an incoming array parameter with the static keyword (I never used it, just seen it in a weird example...)

The ugly similarities between arrays and pointers should not exist in C/C++, modern languages eliminated these to prevent programming mistakes but C/C++ will probably keep them forever because of backward compatibility.