In popular music a slash chord or slashed chord is a chord whose bass note or inversion is indicated by the addition of a slash and the letter of the bass after the root note letter. It does not indicate "or".
For example, a C major chord (C) in second inversion is written C/G, which reads "C slash G". If B was the bass it would be written C/B (making a major seventh chord in third inversion), which is read "C slash B". Some chord may be otherwise not be notated, such as Ab/A. Thus a slash chord may also indicate the chord form or shape and an additional bass note.
In popular music, where the particular arrangement of notes is less important that some other forms, slash chords are generally used only when the specific bass note is important. A common example in guitar based music is in the I-V-vi progression. By placing the third of the V chord in the bass, a descending scale is created in the bass. For example, in the key of G major this would be the chords G, D/F#, Em. In the bass of that progression is the descending scalular line G, F#, E. This type of slash chord contains diatonically occurring notes. In traditional notation it would be written using figured bass symbols.
Slash chords can also be used to denote non-diatonic bass notes, such as a C/F# chord in the key of C Major. These chords are used both for dissonant effects and chromatic basslines. This type of chromaticism is often found in the I-vi-IV-V-I progression between the IV and V chords. For example, in the key of C Major, F, F/F#, G.
Some sources notate slash chords with a horizontal line. (Courtesy of Wikipedia) Click below for the best in free Slash Chord lessons available on the web.