God the Holy Ghost testifies of the father and the son. He testifies of all truth. He comforts. He teaches. He is differentiated as ghost or spirit because he is the only member of the Godhead who does not have a physical body.
The Holy Ghost
Behold, that which you hear is as the voice of one crying in the wilderness—in the wilderness, because you cannot see him—my voice, because my voice is Spirit; my Spirit is truth; truth abideth and hath no end; and if it be in you it shall abound.
People try to say that God is only a spirit based on what they read in John 4:24
24 God is a Spirit: and they that worship him must worship him in spirit and in truth.
DnC 88:66 seems to add a different dimension to that. His voice is a spirit, that spirit being the Holy Ghost.
The Joseph Smith Translation also clarifies this. His translation gives that verse as John 4:26:
26 For unto such hath God promised his Spirit. And they who worship him, must worship in spirit and in truth.