It may seem somewhat academic at this point to define interference but the best understanding starts with a strong foundation in fundamentals.
Webster defines interference as:
a. A jumbling of radio signals, caused by the reception of undesired ones.
b. The signals or devices producing the incoherence.
Meanwhile the Federal Communications Commission (FCC) defines harmful interference as interference, which endangers the functioning of a radio navigation service or other safety services, or seriously degrades, obstructs or repeatedly interrupts a radio communication service operating in accordance with the Radio Regulations.
From the amateur's standpoint, this means you cannot communicate. However, remember that the amateur service is not a safety or essential service, some moderation must be considered. e.g. hearing a distant co-channel repeater when your local repeater is not active, while being annoying, is not "interference". Hearing adjacent channel splatter while carrying on a conversation on simplex or your local repeater while affecting the quality of the conversation is not truly interference. If it makes communication completely impossible then it should be considered interference although it still may not be harmful or necessarily willful.
Take note at this point, that many of the noise sources to be defined below do not affect FM/PM type radio operation except to cause desensing of the radio possibly masking the desired signal.
Taking Webster quite literally, we first examine types of interference:
There isn't much that can be done about these sources.
As you can readily see, there are a multitude of problems waiting for solutions. Each with few exceptions are solvable. It takes patience, perseverance and the cooperation of others.
The natural phenomena as defined above are not within our capability to solve. It is the manmade signals that by and large can be attenuated or controlled.
The biggest thing is don't let these problems beat you, they can be solved. If you need help, there are other amateurs out there that are ready, willing and able to help. Just ask. Remember, sometimes they also could use a hand - it's called sharing the load.
Those are most of the manmade problems now for the difficult ones:
Interference from other amateurs and commercial sources: There are two basic types unintentional or accidental e.g. keying up on the repeater before turning up the volume control. This has happened to many of us and those of you that haven't done it yet will sooner or later. The other type is the harmful, willful or mischievous interference. This is intentionally tying up a repeater or frequency to prevent its use by other persons.