My understanding is streaming is only possible under limited configurations. In other situations the item is downloaded but may start playing once a sufficient buffer has built up so you don't have to wait until download is complete.
iOS: Setting up Home Sharing on your device - https://support.apple.com/en-us/HT202190 - "With Home Sharing in iOS 4.3 or later, you can stream your entire iTunes library over your home Wi-Fi network from your Mac or PC right to your iPhone, iPad, or iPod touch."
http://www.apple.com/itunes/itunes-match/
"Does iTunes Match stream or download songs?
On a computer, any songs stored in iCloud will stream over the air when played, though you can download them at any time by clicking the iCloud download button. iOS devices will start playing tracks from iCloud as they download and will store them so that you can listen to them later even if you don’t have a network connection. Apple TV only streams songs."
stream itunes purchased movies to ipad - https://discussions.apple.com/thread/4120493 You can only stream from a computer using Home Sharing or from iCloud to an AppleTV.