The greenhouse effect is the process that's making the Earth gradually warmer due to the release of gasses such as carbon dioxide. The greenhouse effect is great for hothouse tomatoes, but not so great for the planet.
In a greenhouse, sunlight enters and the heat is trapped inside. That’s a good thing if you’re trying to grow tomatoes in the winter but not so good on the planet as a whole. The greenhouse effect happens when gasses prevent surface radiation from escaping and raise the temperature of the planet slowly. The greenhouse effect is normal, but the vast amounts of fossil fuels we burn are believed to be increasing the greenhouse effect, causing global warming.