The greenhouse effect is a natural physical process where certain trace gasses in the atmosphere, known as greenhouse gasses, warm the earth.
When the sun's energy reaches the earth some of it is reflected back to space and the rest is absorbed. The absorbed energy warms both the atmosphere and the earth's surface, both of which then emit heat back toward space as long-wave radiation. This outgoing long-wave radiation is partially trapped in the atmosphere by greenhouse gases such as carbon dioxide, methane and water vapour which then radiate the energy in all directions, further warming the Earth's surface and the lower levels of the atmosphere. Without these greenhouse gases the Earth's average surface temperature would be about 33 degrees Celsius cooler.
The enhanced greenhouse effect refers to the addition of more heat trapping gases to the atmosphere. The extra greenhouse gases are not 'natural' in origin, and are primarily due to the burning of fossil fuels (coal, oil and natural gas), with smaller contributions from land use changes, such as land clearing. The increased concentrations of greenhouse gases in the atmosphere are contributing to a warming of the earth's surface.