How is the greenhouse effect related to global warming?
Global warming is happening due to the greenhouse effect combined with increased greenhouse gas emissions by human activities.
The greenhouse effect is the process during which radiation from the sun is trapped within the earth's atmosphere, which warms the planet. This effect occurs naturally. Since the industrial revolution, the greenhouse effect has been magnified due to greenhouse gases emitted into the atmosphere by humans.
Global warming refers to the increase in annual average temperatures across the globe. As the amount of greenhouse gases in the atmosphere increases, the planet becomes warmer and warmer on average.
Thus, as humans emit more greenhouse gases and those gases are trapped in the atmosphere, more heat is retained. This causes an average annual warming across the globe.
See this related question on Socratic about the difference between the two.