What are green house gases and where do they come from?

These are gases that trap and store heat.  These are:

Carbon Dioxide (CO2):
Produced when burning solid waste, fossil fuels (oil, natural gas, and coal), and wood.  That means vehicles, power plants, anything that burns!

The natural balance of carbon dioxide in the atmosphere is growing from its stable level of 0.13% to a predicted 0.01% per year. It is anticipated that this extra carbon dioxide will fuel the greenhouse effect, warm the atmosphere, and further disrupt the natural carbon dioxide cycle.

21 times worst than CO2, produced in:

Nitrous oxides:
270 times worst than CO2, produced in:

Hydrofluorocarbons, perfluorocarbons, and sulphur hexafluoride:
These are the super heavy weights, generated in various industries

Does that mean we have less oxygen and nitrogen in the air?

Air consists of 20% oxygen, 79% nitrogen and then 1% other gases.  It is the effect on the 1% that is affecting our environment.  Our oxygen and nitrogen levels have not decreased yet.

What reduces the amount of green house gases in our environment?

Plants and the oceans.

Scientist at Leicester University have been observing the importance of plants in taking out the CO2 from the atmosphere. Plants eat (absorb) carbon dioxide when photosynthesizing (their food making process). So understanding their importance in reducing the CO2 levels is important in today's environmental concerns.

In the ocean there is an amazing process going on here.  CO2 is mildly soluble in water (it makes the fizz in carbonated drinks like soda water).  So the ocean will absorb an amazing amount of CO2 from the atmosphere.  Then these tiny marine plants called phytoplanktons suck up the absorbed CO2 (for photosynthesis).  This sounds good right?  Great CO2 is locked away.  Wrong, other organisms will come along and eat the phytoplanktons.  When they die, or defecate (make "poo") - they release the CO2 back into the atmosphere.  Bad because of the pooing before they die!

Not all bad news, the oceans also contain another small organism called salp.  These guys eat phytoplankton and lock the carbon in their faeces which sinks to the bottom of the sea.  A study on a swarm of salp indicated that 74 per cent of the surface dwelling phytoplanktons were consumed by the salp, and their sinking "poo" pellets transported 4000 tonnes of carbon per day to deep water.

So sounds to me like plant more trees (as we have been told since acid rain days), encourage more phytoplankton growth and be nice to the salps!