MP3 (MPEG-1 Audio Layer III) is the most widely supported audio format in the world, usable on every audio device manufactured in the past 25 years. It uses psychoacoustic compression that removes audio data humans are unlikely to hear — sounds masked by louder concurrent sounds, frequencies near the edge of human hearing range, and very short sounds following loud sounds (temporal masking).
MP3 quality is primarily determined by bitrate: 128 kbps (about 1MB/minute) is standard quality sufficient for casual listening; 192 kbps produces quality indistinguishable from CD for most listeners; 320 kbps is "transparent" — nearly all listeners cannot distinguish it from the original CD. Joint stereo encoding (used at 128-192 kbps) processes stereo information more efficiently than pure stereo.
MP3's main limitation is that it's an older codec — AAC produces better quality at the same bitrate, and OGG Vorbis is comparable. However, MP3's universal compatibility makes it the safest choice for sharing. Use FileCurve to compress audio files to MP3 or trim audio for ringtones and clips.