#2
"Add" chord means that there's a triad with an extension higher than a seventh on top. For example:
C major - C E G
Cadd9 - C E G D

If the major seventh (B) was present, it would be a Cmaj9.

A slash chord indicates that there's a note other than the root in the bass. For example, in C major again:
C major - C E G

Let's say we put an F in the bass of that chord, making it F C E G from bottom to top. That would be called a C/F.
#3
Add chords refers to chord extensions while Slash chords refer to chord inversions.
Proud Owner of:

Jackson RR3
Jackson WRMG

Quote by madbasslover
What's the big deal with Gibsons, anyway?
I've heard loads of Gibsons being played before
and they don't sound any more special than
any other guitar.

^UG's King Of Fail.
#4
Quote by Eternal_One
Add chords refers to chord extensions while Slash chords refer to chord inversions.

It's not always an inversion that the slash refers to, such as the example I gave.
#5
Quote by :-D
It's not always an inversion that the slash refers to, such as the example I gave.



+1 it simply refers to an alternate bass note. They are often inversions, but not always.
#6
I stand corrected.
Proud Owner of:

Jackson RR3
Jackson WRMG

Quote by madbasslover
What's the big deal with Gibsons, anyway?
I've heard loads of Gibsons being played before
and they don't sound any more special than
any other guitar.

^UG's King Of Fail.