96 kbps and 160 kbps. 320 kbps is available in some OS's. Spotify will add the 320 kbps bitrate to all devices, but currently those are in progress. So users, like me, just need more time - when Spotify is updating all those apps.
You can setup sound settings how you like, check app settings. For streaming and offline mode you can setup bitrate you like. For example in EDGE or slow/limited 3G you might be want to try streming quality 96 kbps. Otherwise 160 kbps is good. I use 160 kbps all the time on my mobile, thank god to fast and unlimited 3G network here in Finland by Elisa.
Spotify uses a OGG format and all bitrates are conderted from lossless format.
Spotify uses also VBR intracks.
Hope that this all information you want to know. And ask more, if you have any other questions.