The Standard Library Functions
Some of the "commands" in C are not really "commands" at all but are functions. For example, we have been using printf and scanf to do input and output, and we have used rand to generate random numbers  all three are functions.
There are a great many standard functions that are included with C compilers and while these are not really part of the language, in the sense that you can rewrite them if you really want to, most C programmers think of them as fixtures and fittings. Later in the course we will look into the mysteries of how C gains access to these standard functions and how we can extend the range of the standard library. But for now a list of the most common libraries and a brief description of the most useful functions they contain follows:

stdio.h: I/O functions:
 getchar() returns the next character typed on the keyboard.
 putchar() outputs a single character to the screen.
 printf() as previously described
 scanf() as previously described

string.h: String functions
 strcat() concatenates a copy of str2 to str1
 strcmp() compares two strings
 strcpy() copys contents of str2 to str1

ctype.h: Character functions
 isdigit() returns non0 if arg is digit 0 to 9
 isalpha() returns non0 if arg is a letter of the alphabet
 isalnum() returns non0 if arg is a letter or digit
 islower() returns non0 if arg is lowercase letter
 isupper() returns non0 if arg is uppercase letter

math.h: Mathematics functions
 acos() returns arc cosine of arg
 asin() returns arc sine of arg
 atan() returns arc tangent of arg
 cos() returns cosine of arg
 exp() returns natural logarithim e
 fabs() returns absolute value of num
 sqrt() returns square root of num

time.h: Time and Date functions
 time() returns current calender time of system
 difftime() returns difference in secs between two times
 clock() returns number of system clock cycles since program execution

stdlib.h:Miscellaneous functions
 malloc() provides dynamic memory allocation, covered in future sections
 rand() as already described previously
 srand() used to set the starting point for rand()