Electromagnetic interference (EMI) is a an unfortunate characteristic of radiofrequency energy emission in which radio transmitting equipment is capable of producing.  Conflicting station transmissions, natural properties of radio frequency transmissions, and improperly installed or defective equipment can all be responsible for interference.

THE "LOW POWER" RULE

One of the most sacred creeds of the radio amateur is to always operate on low power if you can possibly do so.  And preventing EMI is one of the reasons for this; the lower the power that you operate on, then the less likely that you are to cause interference with someone else's reception.  When making a contact, always check with your fellow ham to see how clear your signal is coming in.  If it is a very strong signal, then you may want to turn down the power as much as you can without breaking up the signal.  Interference can also be caused by using too much microphone gain or signal processing on a SSB transmitter or shouting into the microphone of any transmitter.  Operating on low power also reduces the risk of exposure to radiofrequency energy and will extend the lifetime of your radio equipment.

HARMONICS

One type of EMI emanates from harmonics, a "nasty" property of all sound and radiofrequency energy in which higher frequencies can unintentionally generated from any given frequency (the fundamental frequency).  To determine what harmonics are possible for a frequency, take that value and multiply it by whole numbers.  The frequency times two is called the second harmonic, times three would be the third frequency, and times four would be, you guessed it, the fourth harmonic.  For example, transmitting on 32.15 MHz generates harmonics of 64.30 MHz, 96.45 MHz, and 128.60 MHz .

Harmonics can cause interference on the ascending frequencies that are adjacent to the one you are operating on.  The harmonic properties of you radio equipment should be suppressed as much as possible to avoid interfering with other amateur stations, and believe it or not, other services such as TV, phone, and broadcast radio. 

FILTERS

A filter is a radio circuit that allows some frequencies to be transmitted and received, but not others.  Let's look at three types of filters that are used in radio equipment to eliminate or reduce EMI.  A low-pass filter allows allows all frequencies below a selected frequency to be transmitted or received, while all others above are cut off.  A low-pass filter could be patched into your radio output to cut off harmonics that may interfere with neighboring service reception.  A high-pass filter is just the opposite - this circuit allows all frequencies above a selected frequency to be transmitted or received, while all others below are cut off.  A device like this could be used on a neighbor's television if you have had a problem with harmonic radiation to keep you from interfering with his reception.  A band-pass filter is used to cut out frequencies on either side of a selected band.  Band-pass filters are used in transceivers to ensure that hams don't accidentally transmit outside of a selected band, like the two-meter range (144 - 148 MHz).  That keeps you from coming in on frequencies allotted for other services.

Receiver overload is a problem that must be considered - it is caused by your radio equipment interfering with the reception of other radio devices (these can belong to you or your neighbors) due to close proximity.  If there is a problem with your radio equipment interfering with your neighbor's reception, it is the amateur's responsibility to solve the problem, even though the problem may very well stem from the shoddy shielding of his cheap TV.  But you are the radio "expert" (relatively speaking, that is) and as a responsible amateur, I encourage you to do everything you can to rectify the situation.  There are filters available that the owner, or his service provider, can utilize to prevent interference from occurring, especially if it is of harmonic origin.  But it may be that you have a problem somewhere in your set-up, possibly a break in the shielding of your feedline, a poor connection, or a poor selection of coax for the bands that you are operating on.  First make sure that your radio equipment is not interfering with your TV reception.  It's important to "treat the disease", not "the symptoms".  So carefully diagnose the problem before selecting a solution.  Also, remember that VHF transmissions can interfere with cable TV if connections are loose or if the line is damaged.

REPEATER COORDINATION

Frequency coordinators are responsible for determining the available frequencies when setting up a repeater.  A system is used to ensure that repeaters operating on the same frequencies are placed as far apart as possible so that they won't interfere with each other.  Amateur repeaters should all be coordinated; if an uncoordinated repeater interferes with a coordinated one, guess who is responsible for resolving the conflict?  The licensee of the uncoordinated repeater.  If both are coordinated, or if neither have been coordinated, then the responsibility falls to both licensees.

"HARMFUL INTERFERENCE"

Harmful interference stems from intentionally jamming someone else's communications by transmitting on a frequency that one is fully aware is currently occupied.  Don't do this, even as a joke.  And if you know anybody that is, report them to the FCC - it's not right, and it messes it up for everybody.  These vagrants are subject to having their licenses yanked, and if they are jamming public safety frequencies, they can be subject to severe penalty.