In one of my instruction books, and on several web pages, I am instructed to make the E minor 7th chord by putting my 2nd finger on the 2nd fret of the A string; however, the Ultimate Chord Finder indicates that I'm also supposed to put my 4th finger on the 3rd fret of the B string (note D).

To make things even more confusing, I found yet another site that instructs me to add my 3rd finger on the 2nd fret of the D string (note E), in addition to the other two fingers.

I'm very confused, especially because these 3 different methods produce very different sounds. Which is correct?

Thanks!