One throws away data to save space, the other doesn't throw anything away at all. Here's when that difference is worth caring about.
MP3 is lossy: it analyzes the audio, decides which parts a human ear is least likely to notice missing, and throws them away to shrink the file. FLAC is lossless: it compresses the file the way a zip archive does, so decoding it gets you back the exact original waveform, bit for bit.
Both formats have been around since the 1990s, and neither is going anywhere. The question isn't which one is objectively better, it's which tradeoff fits what you're doing right now.
| MP3 | FLAC | |
|---|---|---|
| Data | Lossy, some audio discarded | Lossless, bit-for-bit exact |
| File size | Small, roughly a tenth of WAV | Large, roughly half of WAV |
| Best for | Phones, earbuds, limited storage | Archiving, critical listening, good headphones |
| Can convert to the other losslessly | No | No |
On a phone speaker or a subway platform, most people cannot reliably tell a well-encoded MP3 from FLAC in a blind test. On a decent pair of headphones in a quiet room, on a proper set of monitors, the gap becomes real for a lot of listeners, especially on dense or dynamic recordings. Archiving matters too: rip once to FLAC and you can always make an MP3 from it later. Rip straight to MP3 and that decision is permanent.
Converting an MP3 to FLAC does not recover the audio that was already discarded. It just wraps the same reduced-quality data in a lossless container, producing a bigger file with no quality gained. If archival quality matters to you at all, rip to FLAC first and generate MP3 copies from that master when you need something smaller, never the other way around.