While the ultimate indicator of quality is what sounds best. If you're looking for the "cleanest" sound, there are a few things you should look for.
Typically, you want to find audio files in unaltered formats - though it comes at the cost of dramatically larger filesizes. These are audio formats (both compressed and uncompressed) that store the audio data without changing the content inside. Common filetypes are '.wav', '.aiff', '.m4a', and '.flac'. "Lossy" files are those that contain audio that has been altered to be more easily compressed into a small filesize. Common filenames are '.mp3', '.aac', '.m4a', '.wma', and '.ogg'.
'.m4a' is a bit special because both lossy and lossless files can be encoded in this format, so it's a gamble whether or not you'll get unaltered files.
More information on filetypes here:
Audio file format.
Lossy encoders generally reduce the complexity of the sound, and thus - technically - the quality, though it does this in hard to discern ways. Most use variations of a psychoacoustic masking trick that removes certain pieces of audio without the listener noticing. Quiet frequencies in audio that are near loud frequencies can be cut out, since they're sounds that are typically very difficult to hear normally. They will also cut out quiet transients (transients are loud volume peaks in an audio track, for instance the initial snap of a snare) that are nearby loud transients, since most people will only hear the the loud transient even if the quiet one came before it.
This effects some major aspects of the sound. Lossy encoders will muddy the stereo image, as the phase and frequencies of audio have been altered. This means that altered stereo orchestral recordings, for instance, can sound flat or hollow. You'll also get some noticeable artifacts for noisy sounds, like cymbals. You may have noticed that in compressed audio, the cymbals might have become more hissy than before. You may also lose some low frequency definition, especially sharp bass attacks, since bass frequencies can be hard to differentiate from transients.
There's a much more in-depth discussion of the effects of lossy encoding here: What Data Compression Does To Your Music.
Frankly, most people don't care about the quality of the samples that you use. And, if you put any amount of reverb on the sound, Few people will ever notice.