iTunes Match is Apple's "music locker" service that allows you to store copies of all your music on Apple's servers and re-download it from any of your devices, at any time. iTunes Match isn't free, however. It comes at a price -- $24.99 a year (in the U.S.). It's also only available in 20 odd regions right now.
There are a few limitations to iTunes Match which you should be aware of before subscribing.
iTunes Match isn't available everywhere yet, only a few reasons. Check your iTunes Match availability by country
iTunes Match can only handle 25,000 songs. If your library is bigger than that, iTunes will refuse to match it. About all you can do right now is create a new iTunes library on the same computer with a subset of your songs -- no more than 25,000 -- and use that library to set up iTunes Match. (Just make sure you don't copy your music into new folders, leave it in its existing folders or you'll waste a lot of disk space.)
iTunes Match doesn't always do a great job matching every of songs. It could be an issue with metadata, if the song information is really different between the iTunes Catalog copy and your local copy. You could try updating the metadata on your local copies before going through iTunes Match, but it can take a lot of time and results may still vary. (Anything iTunes doesn't match gets uploaded to Apple's servers, so you still have access to it, but in its original form, not the iTunes Catalog copy in 256Kbps AAC with "official" metadata)
Next, iTunes Match will go through a three step process:
Depending on how much music needs to be uploaded, and the speed of your internet connection, it could take a while to complete.
Once you've subscribed to iTunes Match, you can easily set it up on your iPhone, iPod touch, or iPad.
If you don't see the iTunes Match option, make sure you're logged into the same iTunes account with which you subscribed to iTunes Match. If you still don't see it, scroll down to the bottom of the Music Settings and enter that iTunes ID into the Home Sharing login. That should wake it up.
iTunes Match can either be Wi-Fi only, or can also let you re-download music over 3G/4G connections.
This will make sure that your Music app shows you every song in your iTunes Match library, whether you've downloaded it to your iPhone, iPod touch, or iPad or not.
If you still need help with subscribing to, setting up, or using iTunes Match on your iPhone, iPod touch, or iPad, or any other iCloud feature, head on over to our iCloud Forum and ask away!
Ally Kazmucha, Chris Oldroyd, and Gary Mazo contributed major sections of this guide.