One good piece which helps me when starting with C (taken from the Expert C Programming by Linden)
The One 'l' nul and the Two 'l' null
Memorize this little rhyme to recall the correct terminology for pointers and ASCII zero:
The one "l" NUL ends an ASCII string,
The two "l" NULL points to no thing.
Apologies to Ogden Nash, but the three "l" nulll means check your spelling.