http://support.apple.com/kb/HT4914?viewlocale=en_US&locale=en_US

Quote:
About file formats and iTunes Match


Songs encoded as MP3 or AAC that cannot be matched to the iTunes Store will be uploaded as is. These songs will be made available for download in the same format it was uploaded in.
Songs encoded as MP3 or AAC that do not meet certain quality criteria will not be matched or uploaded to iCloud. (recorded below 96kbps)
Songs encoded as ALAC, WAV, or AIFF, will be (temporarily) transcoded in iTunes to 256 kbps AAC when uploaded to iCloud.
Song files over 200 MB will not be uploaded to iCloud.
Matched songs in your local library will not automatically be transcoded to 256 kbps AAC. (The quality in your library will remain the same)


iTunes Match is limited to 25,000 songs.
iTunes Store purchases made with the same Apple ID being used for iTunes Match do not count towards the 25,000 song limit.
Song files over 200 MB will not be uploaded to iCloud.
Songs containing DRM (Digital Rights Management) will not be matched or uploaded to iCloud unless your computer is authorized for playback of that content.

About file formats

Songs encoded as MP3 or AAC that have been matched to the iTunes Store will be made available for download as 256 kbps as AAC from iCloud.
Songs encoded as AAC or MP3 that do not meet certain quality criteria will not be matched or uploaded to iCloud. (recorded below 96kbps)
Songs encoded in ALAC, WAV, or AIFF will be transcoded to a separate temporary AAC 256 Kbps file locally, prior to uploading to iCloud. The original files will remain untouched. To obtain a higher quality transcoding, download the matched songs from iCloud.
Songs encoded as MP3 or AAC that cannot be matched to the iTunes Store will be uploaded as is. These songs will be made available for download in the same format it was uploaded in.


I added the bold for clarity.

All matched music plays back at 256 kbps AAC.

Available on all devices, up to 10 devices. You do have to have an internet connection for it to stream of course. You can choose to store some locally on your devices also for when not around internet.