On the web, we stream in 320kbps MP3 quality audio - industry standard high quality audio for streaming services. A small minority of tracks may not be available for 320, in which case we'll stream in 256kbps.
On mobile, you have some options. You can choose to stream in standard quality audio - which is HE-AAC 64kbps. Or you can choose high quality audio, which is 320kbps MP3.
Note that streaming in high quality audio uses more mobile data when you're not on wifi. You can also choose to only stream in high quality audio when connected to wifi.