Happy Cards are multi-store gift cards. Happy Guy, Happy Lady, Happy Bride, etc. that cluster about 6 stores that might be of interest to a certain type of recipient. The thing that sets them apart from regular gift cards is they run as credit. Basically, they're like a pre-paid Visa that can only be used at a few stores. Some smart people realized that since they run as credit, they can be used to buy other gift cards (like Disney gift cards) even at stores that do not allow gift card to gift card purchases.
So, if you buy a Happy Guy card, you can go to Home Depot and use it to buy a Disney gift card. However, if you have a Home Depot gift card, you cannot buy a Disney gift card with it because Home Depot does not allow gift card to gift card purchases.
They're basically a loophole. When you buy a Happy Card, you buy it as a gift card. When you spend a Happy Card, you spend it as a Visa. Since you're buying Happy Cards as gift cards, you can often buy them at discounts you'd never see for a prepaid Visa. Since you're spending Happy Cards as credit cards, you can use them to buy whatever gift card you want.
Make sense?