This is an extended chord played with both the seventh (or sixth) and ninth added. The ninth is the note nine scale degrees from the root of the chord (or the second scale degree one octave higher). Thus, built on a root of D, a dominant ninth chord consists of the notes D F# A C and E. You could also add a ninth to a D major 7th chord (D F# A C# E), D major 6th (D F# A B E), D minor 7th (D F A C E), D minor 6th (D F A B E), D minor 7th b5 (D F Ab C E), or any of a variety of other seventh and sixth chord structures.
A ninth chord that’s commonly referred to as the “Hendrix chord” is an extended dominant chord with a sharped ninth. (Built on E, it would be E G# B, D, F## – F-double-sharp is enharmonically/sounds the same as a G, but technically, in order to function as the 9th of E it has to be spelled as an F## instead of a G.) The best example of this is in the Hendrix hit “Purple Haze.”
There is also the flat ninth dominant 7th chord. (Built on E, it would be E G# B D F). This chord is common in jazz compositions such as “Stella By Starlight”, usually preceeded in the progression by a minor 7th b5 chord 5 scale notes below (or 4 scale notes above) the root of the dominant 7th #9 chord. So if you were going to use a E7#9, try playing a B minor 7th b5 going to E7#9 and finally resolving to A minor, A minor 7th, or A minor 9th.
A ninth chord is different from an “add 9” chord, which simply adds the 9th scale degree on top of a triad (D F# A E), or a “sus 9” or suspended 9th chord (also known as a “sus 2” chord), where the 9th scale degree replaces the third of a triad, forming a suspension. (D A E)
A common application of ninth chord extensions is on major and minor 12-bar blues progressions.