From the term "metamorphosis," it is to turn one thing into another. It most commonly refers to special effects morphing and it is the animated transformation of one image into another, by a gradual distortion of the first image. Certain points of the first image move to the position of corresponding points in the second image.
Popular culture has adopted the term "morph" to refer to anything that has changed from one thing to another. For example many terms in NetLingo, including this one, originally described a certain meaning, and later came to describe a new or additional meaning, (or a new grammatical way of using the term).
The term morph also refers to the technique spammers use when they alter the header of an e-mail to avoid anti-spam software.