My favorite explanation is by Graeme Nattress: When the lens resolves more detail than the sensor can capture and that extra detail is *not* filtered out by an AA OLPF, you're trying to fit more resolution than the pixels dimensions can handle. In real life when you put more than a pint of beer in a pint pot, the beer spills out and makes a mess. In image processing, the beer folds back on itself and corrupts the image - aliasing, but just another mess of beer really. Aliases are spurious false data that are not correlated with the actual detail in the image. They're effectively noise, or extra detail that's not actually part of the image.
I would add that a lot of photographers like aliasing artifacts. Most that shoot Sigma Foveon, for example, call the images "crisp", "sharp", etc., but what they're actually seeing is aliasing. You can even get aliases with a camera that has an OLPF, by just downsampling with a bad algorithm, such as nearest neighbor.
To me, aliasing looks fake and computer-generated, like fonts that are not anti-aliased. Correctly filtered images appear more natural and desirable to my eye.