WHAT EXACTLY IS HAPPENING?
Apple will soon be splitting iTunes into specific applications tailored towards each product. Instead of having it all under one roof, now TV (includes Movies), Music, and Podcasts will each have their own apps where you can browse, listen, watch, and purchase content. For more details, see this article.
DOES THIS AFFECT ME?
NOTHING IS CHANGING WITH GIFT CARDS. Aside from content being in different applications now, nothing else will be different for you. You will still be able to buy App/iTunes gift cards, redeem them to your Apple account, and continue to use all of Apple’s services just like before. The only difference is there will be different apps for different content.