Cell phones are in constant contact with cell towers communicating various things like its identification number.  Cell phone towers have a range of communication of around 25 kilometres.  They are spaced around 20 kilometres apart and their area of range is called a cell.  Hence the term cell phone.  When you make a cell call the phone connects to the tower sending a request to the towers computer to locate the identification number of the phone you dialled.  Once this is located the tower connects to the tower of the phone you called telling it to establish a connection with the phone you are calling making it ring.  Once the ring is answered the two cell phones are put into communication with each other and talking can begin.

The cell phone jammer works to prevent any contact with the cell phonetower.  The jammer saturates and floods the signal range that the cell system uses with more power than cell phones emit and the phone cannot establish any connection with the cell tower.  This prevents any communication within the jammers range.