Originally Posted by: Sally SteeltoeIn 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).[/quote]
Every chord has a formula, a specific group of notes from it's parent scale. To make a minor 7th chord, you need 4 notes from the parent minor scale.

To make an E minor 7th chord you need 4 notes from the E minor scale:

1st (E)
minor 3rd (G)
5th (B)
minor 7th (D)

All minor 7th chords use that formula: 1st note, minor 3rd note, 5th note, minor 7th note.

Once you've got those 4 notes you need to play them on your musical instrument! Because there are many places to play any given note on the guitar (or piano, etc.), there are many ways to play any given chord. In fact, anywhere on the guitar you can find to play those 4 notes is another variation on an E minor 7th chord.
[QUOTE=Sally Steeltoe]I'm very confused, especially because these 3 different methods produce very different sounds. Which is correct?

So, as long as they all have the notes E, G, B, and D, they are all correct! The question is, which one sounds the best for the song, or your musical intention?

FYI, these tutorials cover basic chord theory & major & minor 7th chords:

http://www.guitartricks.com/tutorial.php?input=824
http://www.guitartricks.com/tutorial.php?input=501

Hope this helps, ask more if necessary!