Choose any audio file your browser can decode and press Convert to WAV. The file is decoded locally, written to a proper RIFF/WAVE container and offered as a download.
No. MP3 compression discards information permanently, so the WAV is a faithful copy of the decoded MP3 rather than of the original master. WAV is still useful because it edits and imports cleanly without repeated lossy re-encoding.
A standard RIFF/WAVE file with 16-bit signed PCM samples at the source file's own sample rate and channel count. That combination opens in every editor, DAW, game engine and operating system without extra codecs.
WAV stores every sample uncompressed, roughly 10 MB per minute of stereo CD-quality audio, while MP3 throws away most of that data. The size is the price of a format that never degrades when you edit and re-save it.