OK, a couple of different things.
The IDE
First, SmartThings has multiple “shards” (can think of those like cloud-based database servers) and different accounts are associated with different ones.
SmartThings will let you sign into a shard not associated with your account, but when you do you won’t see any of your devices. 
(You would think smart things would route you automatically, but it doesn’t)
Because there are several different ones, it’s best not to just follow individual links from forum posts as the person posting might be on a different shard than you are.
The following FAQ lists the shards that exist at the present time:
FAQ: How to find out what "shard" (cloud slice IDE URL) your Account / Location is on?
What’s a Device Handler?
- as far as which device type handler you are using, if you just use the stock one when you added the device, you don’t have to worry about going into the IDE it all for now.
But to clear up the confusion, smart things terminology has changed several times. All of the following mean the same thing:
Device type
Device handler
Device type handler
DH
DTH
As for what they are, the custom code FAQ explains the concepts. You won’t need to follow any of the steps because you’re not using custom code, but it’s a good overview of what people are talking about so you might want to just read the first part anyway.