There are many types of interference. Some examples are co-channel (someone else, generally some distance away, using the same frequency as you) and adjacent channel interference (signals on a frequency different from but close to your desired received frequency) & RF LEAKAGES. Other examples include manmade noise from induction devices such as motors and internal combustion engines as well as spurious emissions from non-radio devices such as computers, medical devices and welding equipment.
Whenever two or more signals are present in a non-linear device, The possibility exists that an inter-modulation product will be spontaneously generated as some combination of sums and/or difference between the frequencies involved. The contributing frequencies can be fundamental carrier frequencies radiated from transmitters operating in the nearby vicinity.
They can also be harmonic products of those carriers (twice the frequency, three times, etc.)
Which may also be radiated if the proper suppression equipment is not installed or properly tuned at the transmitter output.