God compares Jerusalem and Samaria to two young sisters who both grew up to become prostitutes selling themselves to foreign gods:
The word of the Lord came to me: Mortal, there were two women, the daughters of one mother; they played the whore in Egypt; they played the whore in their youth; their breasts were caressed there, and their virgin bosoms were fondled. Oholah was the name of the elder and Oholibah the name of her sister. They became mine, and they bore sons and daughters. As for their names, Oholah is Samaria, and Oholibah is Jerusalem. (23:1-4)
The names Oholah and Oholibah mean, respectively, "her tent" and "my tent is in her." In talking about the adultery these two sister-wives commit against him, God constantly draws an analogy between adultery and idolatry. Idolatry's the national or social equivalent of cheating on your spouse. It's the way nations become unfaithful.
The analogy in this case gets pretty graphic—God takes it as far as it can go, really—particularly in verses 23:20-21. In his book Seeking Ezekiel , the psychoanalyst David Halperin suggested that these verses show that Ezekiel had some serious issues with women and sex. He saw a dread and fear of women's sexuality as behind many of Ezekiel's visions. Think about that vision where he blasted through a "wall" to find some disgusting things and see if you think Dr. Halperin has a point.