Although most people probably know this, its important to have a common understanding.
So in an attempt to explain the situation in as few words as possible.
The Earth receives energy from the Sun which in turn heats up the air, land and oceans, this heat is transmitted to the atmosphere. This heat is eventually released into space. For the average temperature of the Earth to be constant the amount of energy received has to be equivalent to amount of energy dissipated.
The gases CO2, Methane CH4, Nitrous Oxides NO NO2, Sulphur Oxides SO2 among others, trap the heat in the atmosphere, interfering with the delicate balance. They act similarly to the glass in a greenhouse, and the gases are known as greenhouse gases.
Its part of life that animals and fish give off CO2 and that plants and trees turn CO2 into O2 without this there would be no life on Earth.
Since the Industrial Revolution we have been increasing the amount of greenhouse gases released into the atmosphere, and at the same time cutting down trees without planting enough new ones to compensate. The existing plants and trees are not able to consume all the extra CO2 that we are releasing, and so the amount of CO2 in the atmosphere is rising. Also the Sulphur Oxides released, react with water forming acids, that damage plants. Methane is particulaly worrying as it stays in the atmosphere for a very long time, the time taken for Methane to disintegrate is around 100 years. (Its really the half life but lets keep it simple)
The net effect of the rise in greenhouse gases is that the Earth is warming.
Scientists predict a catastrophic climate change if the Earth warms by more than 2 degrees.
That does not sound like a lot.