Global warming is a slow steady rise in Earth's surface temperature. Temperatures today are 0.74 °C (1.33 °F) higher than 150 years ago.
The basic cause seems to be a rise in atmospheric carbon dioxide, as predicted by Svante Arrhenius a hundred years ago. When people use fossil fuels like coal and oil, this adds carbon dioxide to the air. When people cut down the Earth's forests (deforestation), this means less carbon dioxide is taken out of the atmosphere by plants.