Note: If you were looking for some magical chords that will make you sound like the guitar player Slash formerly of Guns 'N' Roses, I'm sorry, there are no magical chords, just magical players. This is an explanation of what Slash Chords in musical notation are and how and when to use them. It's almost magical.
A slash chord is simply a chord that uses a note other than the root note as its bass (lowest pitched) note. Since guitar chords often have multiples instances of the same note in a chord, and the root is not always the lowest pitch (bass) note, slash chords are more commonly used by piano players. But guitarists need to understand them and know how and when to play them.
To really understand slash chords you need to understand inversions. In a standard Major triad chord you have the formula: Root, 3rd, 5th. A C major chord would be the notes C (root), E (3rd note in the C Major scale), G (5th note in the C Major scale). The C note is the lowest pitch note.
If you take the C note and make it the highest pitch note, you still have the same three notes (C, E, G) but you now have an E as the lowest pitch note (E, G, C.)
This is called an inversion. This is often written as a C/E slash chord. The letter on the left tells you the chord name, and the letter on the right tells you the bass (lowest pitch) note. If you play a standard open C chord on guitar, strumming all six strings, you are really playing a C/E chord because the bass (lowest pitch) note is an E note (open 6th string.) Although it is usually just written as a C chord.
To Play Or Not Play Slash Notes
Because the guitar has six strings, "inversions" are a common element of guitar chords. Here are the notes in a common open C chord on guitar:
1st string: E
2nd string: C
3rd string: G
4th string: E
5th string: C
6th string: E
You are actually playing multiples of the three notes that make up a C chord (C, E, G) in different octaves. Isn't music theory fun!
Typically you are safe ignoring the bass note in a slash chord if it is one of the notes that make up the chord. Or in other words, if it is an inversion of the chord. Sometimes, however, you might want to emphasize a bass note (such as a moving bass line while fingerpicking) and in this case you should make the slash note the lowest note.
Adding Notes Not In The Chord
The slash note does not have to be an inversion. It can be a note not found in the chord. For example you could add a G note to an A Minor chord to get an Am/G chord. In this case you have to consider the musical context and your playing situation. If you are playing solo guitar you will almost always want to play the slash note. If you play an open A Minor chord and use your 4th finger to grab the G note you have done it.
However, if you are playing in a band situation, you may choose to let the bass player play the G “slash” note and play a standard Am chord. It really depends on what sounds better and what you can physically play.
Common Open Slash Chords
Here are some common slash chords used by guitar players.