What it is
A FLAC file is compressed audio that decodes back to the samples that went in. It opens with the four letters fLaC, then a run of metadata blocks, then the frames holding the audio.
Lossless is the whole claim. A quarter second of 44.1 kHz mono goes from 22,094 bytes as a WAV to 11,794 bytes as a FLAC, and decoding it returns the original sample values. Nothing is approximated the way it is in an MP3.
fLaC STREAMINFO 34 bytes required, and always first VORBIS_COMMENT optional tags, as text SEEKTABLE optional positions for fast scrubbing frames the audio
What is inside
STREAMINFO is required and comes first. Alongside the usual numbers it stores an MD5 of the decoded audio, which is what lets a FLAC be checked for damage in a way a WAV cannot.
sample rate 44100 channels 1 bits per sample 16 total frames 11025 MD5 16 bytes, over the decoded audio
Vocabulary
Lossless is not the same as uncompressed. A WAV is uncompressed and a FLAC is compressed, yet both hand back identical audio. The word that means quality was discarded is lossy, and that is MP3, AAC and Ogg Vorbis.
FLAC stands for Free Lossless Audio Codec. The format is open and the reference encoder is free, which is most of why it won over Apple Lossless outside the Apple ecosystem.
What reads it
Most desktop DAWs open one directly: Reaper, Bitwig, Renoise and Ableton Live among them. Support across hardware samplers is far thinner, and an SD card full of FLAC is a reasonable way to find that out the hard way.
Hardware tends to want PCM because decoding costs CPU and memory at exactly the moment a sampler has neither to spare. A box streaming voices from a card reads WAV for the same reason it reads 16 bit rather than 32.
Where the loop points go
There is no agreed place in a FLAC to record a loop. WAV has its smpl chunk and AIFF has MARK with INST, while FLAC has neither, so a loop that a sampler wrote into a WAV does not survive the trip.
Vorbis comments can hold anything you like as text, and various tools do exactly that. Nothing reads anyone else's convention, so in practice the loop is gone.
The limits
Sample Miner re-embeds loop points after conversion only when the output is WAV. Exporting the same instrument as FLAC produces correct audio with no loop, which is fine for one shots and wrong for anything sustained.
The compression buys roughly half the size on musical material and close to nothing on noise. A library of short percussive one shots compresses worse than the ratio on an album would suggest, because there is less of a signal for the encoder to predict.