When you turn on iTunes match on your iphone, you can see every song in your iCloud library in your iphone, ipad, or ipod touch music app. You can then either stream or download the music onto your idevice. Its a great way to have instant access to your entire music library without having to constantly resync new music onto your iphone when you want it.
I've noticed, however, that I end up downloading songs that I listen to frequently to avoid using extra data. Eventually, those downloaded songs begin to take up a lot of space on your iphone, and it can be difficult to find and delete downloaded songs because they are mixed up with icloud songs.
Here's an easy way I found to show only songs that you have downloaded onto your iphone. In settings, choose music, then turn off "show all music." When you reopen your music app, you will only see songs that you have downloaded into your iphones memory. You can then select either songs, or albums, and swipe your finger left to right to delete those from your hard drive. This will clear up your iphone's memory to use for additional storage.
You can then go back into settings and turn on "show all music" to see your entire iCloud library again.