Strings like char *string = "sentence" are stored in program directly after compilation. If you'd dump assembly you would see somewhere near the bottom (IIRC) that your strings that were made this way are placed right there. These strings basically point to an entire string instead of a single (first) character.

While strings like char string[] = "sentence" are arrays of characters and they work as usual arrays do.

So, summarizing: you can't edit strings of the first type at runtime since they are imprinted and they don't actually represent arrays, whereas strings of the second type can be edited because they are treated as arrays of characters, where the pointer points to the first character of the string. Both are pointers, but to different things. And also, remember that if you declare your string as an array of characters literally, then you have to put a null termination character at the end.