Ivan, 78% of our atmosphere is nitrogen gas, N2. The nitrogen cycle outlines the movement of nitrogen through its different forms as it travels between air, land and water.
The N2 in the air is in a form not usable by plants and animals so that is where bacteria help out. These are nitrogen fixing bacteria and they convert N2 to ammonia, NH3. Other bacteria (nitrifying bacteria) convert ammonia to nitrites and nitrates. Plants use these forms of nitrogen and animals eat plants and thus nitrogen gets incorporated into bodies of living things (proteins and nucleic acids contain nitrogen).
Now how do we get nitrogen back into the atmosphere (remember this is a cycle). Bacteria again! This time denitrifying bacteria convert nitrogen back into N2 form and we are back where we started.