Ever heard of 'Tour de France'??
actually... To summarise, its:
Fish produce waste, which is in the form of ammonia. Ammonia is very bad.
Eventually, Bacteria will build up in your tank (Bacteria is good) and break down the Ammonia, it then turns into Nitrites (Also very bad). Then after a while the bacteria breaks the nitrites down into NitrAtes (notice different spelling) Which is good for plants etc... But only good in small doses....
Once the bacteria colony is big enough, it breaks down the Ammonia, into Nitrites, then into NitrAtes so fast, that neither the Ammonia or Nitrites can effect the fish! To stop the nitrAtes getting too high, you do water changes on a regular basis.
A fishless cycle, can take around 10 days to 2 weeks. A cycle with fish, takes much longer (plus they are exposed to the bad levels of Ammonia and Nitrites!)
I hope i answered your question ok....
