I've done the research regarding the definition of those words that I took from a dictionary.
an action, an object or a system by which a result is achieved; a way of achieving or doing something
means of something Email is a highly effective means of communication.
a thing that helps you to do your job or to achieve something
An email newsletter can be a very effective communication tool.
By the way, I took the examples from the dictionary I'm using (OALD) as well.
As you can see there, both are similar (to my perspective). Can you tell me the significant difference between these 2 words?