The tank cycle is the biological process of taking the fish execrement which is high in amonia and ultimately deadly, and changing it through the natural biological process to NitrIte and then to NitrAte. Once you put living fish in (or even plants sometimes) the water Amonia level will rise dramatically. A bacteria will naturally develop and turn it into NitrIte over the course of about 3 weeks. Then a second bacteria will form and turn the NitrIte into relatively harmless NitrAte.
Go get an Amonia and a NitrIte test kit. Test daily and keep a chart and you will see it happen. Keeping bubles going in the tank will help speed it up. Water changes slow it down but keep the fish alive durring they cycle. The whole thing is usually about 4 to 6 weeks. An excellent artical on this is at <a href="http://www.bestfish.com" target="_blank">www.bestfish.com</a> it's worth reading but come back here for everything else.
Erik