exactly? they tell you the file size.

no, seriously. the bitrate tells you the rate of bits per second (usually annotated in kbps = kilo bits per second). given a song length in seconds it tells you the final file size.
if we have a song of 2min = 120s, at 320 kbps the file will be 38,400 kilobit, divide by 8 for kilobyte: 4,800 kB large (give or take with headers, etc).

what it means in terms of audio quality is far far more murky. one thing is clear: within a given codec (eg mp3) bigger numbers = better quality. the more bits per second are available for audio information, the more audio information can be stored and the better (ie more like the uncompressed original) it will sound.
the trouble is, you're literally trying to compare apples with oranges if you want to cross reference the bitrates of different codecs. the more advanced a codec is, the better its compression is and therefore the less bitrate it needs for a similar quality.

mp3 and aac are both relatively old codecs by now. aac compresses slightly better (beware: once you get to the audio quality part, the standard is a subjective one - this is just how i hear it!), so i would say 320 kbps mp3 is pretty much equal to 256 kbps aac.
more modern codecs are game changers. Vorbis, part of the second to last generation, sounds equally good with ~120 kbps. Opus, part of the latest generation, probably needs even less (i haven't really worked with Opus yet, but Google does on YouTube).

>>137078
thanks for the info anon. what is the future of mp3? it's pretty popular these days, i haven't even heard of opus or vorbis, i don't even know what their extensions look like. is flac considered old?

>>137098
well, mp3 will always exist (the same way ancient real media files from the dawn of the web will be playable). i have to assume it'll be replaced as a widespread medium one day, the same way xvid isn't used as a video codec anymore. for now it's far too popular to just shut it down (you are the best example, most people think mp3 is a synonym for music). maybe the difference is that with video codecs the visual and file size improvements are *massive*, while with audio you'd just save a few MB per song, which nobody cares about.

the other thing is, mp3 and aac are so old and popular, that most computers and smartphones have specialised *hardware* decoders. their CPUs are literally optimised to very efficiently decode mp3 and aac. so the codecs are probably not dying very soon, altough they have long passed their prime. battery life is an issue with rising mobile use, so chances are, whichever codec manages to get hardware support first will be the new mp3.

mp3 is from 1993, aac from 1997, vorbis from 2000 and opus from 2012. let's see when we manage to get rid of the dinosaurs :-) btw, vorbis is the audio codec in the webm specification, so if you ever clicked on a webm with sound on /gif/ or /wsg/ you've been using vorbis already.

flac is another beast entirely. so far we've been talking about lossy codecs. that means the files are smaller due to compression, but some audio data is lost along the way. codecs try to cut out the stuff that is less noticable to humans. flac on the other hand is a lossless codec. no information is lost. you can convert it back to the uncompressed original and it should be the same. it just compresses what it can to reduce file size a bit.
so far it seems to be hear to stay, no serious competitor has risen as far as i know. it's also free and open, so everyone can implement it (but since there is no industry group pushing it, it's very unlikely to receive hardware support).

>>137113
thanks anon :]

>>137113
>no serious competitor has risen as far as i know
ALAC has hardware support on iOS.

But the point is kinda moot, because you can losslessly transcode between lossless formats.

>>137209
>serious competitor
>apple

i take that as a joke, but i hate that you're probably right