NULL is a character or pointer value. If a pointer, then the pointer variable does not reference any object (i.e. a pointer to nothing). It is usual for functions which return pointers to return NULL if they failed in some way. The return value can be tested. See the section on fopen for an example of this.
NULL is returned by read commands of the gets family when they try to read beyond the end of an input file.
Where it is used as a character, NULL is commonly written as '\0'. It is the string termination character which is automatically appended to any strings in your C program. You usually need not bother about this final \0', since it is handled automatically. However it sometimes makes a useful target to terminate a string search. There is an example of this in the string_length function example in the section on Functions in C.