Ghetto is not actually a slang word. It expresses in general the idea of a minority community that is separate from the majority culture, but it also suggests that the minority community is poor, isolated and that the living conditions are crowded and undesirable.
In the case of American English, it refers to inner city African American communities. The word is not used so much anymore, but when I was growing up in the 1970s it was a very common term. The ghettos of the time were often centered around housing projects that concentrated people, who were always poor and nearly always black, into small neighborhoods.