Are Chives and Green Onions the same thing?
They're not the same thing, as they're a different plant, but they're part of the same family. You might be able to substitute the tops of green onions (scallion, spring onion) in place of chives in small quanities.

Chives tend to be smaller and more tender, typically used raw, and only the green portion is used.  Green onions often use both the whites and the green part, has a more pronounced flavor, and is used both cooked and raw.

So, if you're going to substitute chives for green onion -- use the greens only, and reduce the amount.

If you're going to go the other way, you'll want to look to see what parts are being used.  If it's the whites of the green onion you need, use regular white onion, if it's the greens, then you can try chives, but add them as late as you can in the cooking process, and if it was to be used raw, you might need to increase the amount.