Both can be used, it depends on the context.
I would say 'words' if you are quoting someome, or referring to what they said. For example, 'His words will echo through eternity.' Or, 'Harry's words were full of wisdom.'
'Word' would be used if you took the words someone spoke as singluar. For example, 'Tom gave his word not to tell anybody.'
Like I said, not a teacher, hopefully a teacher may be able to clarify if I haven't explained it very well.
Student or Learner