Let Us C Solutions, chapter 10, Strings. A string in C is nothing but an array of chars. The end of the string is determined by the terminating null character '\0'. So if a string contains "Hello" so it actually containing 6 characters, 5 are each alphabet, and one is '\0'.
This chapter has string concepts of pointers, array, and memory management. It includes the library functions of String.h for utilities like concatenation of two strings, changing the case, etc.