Bitcoin Wallet Creation I'm new to this Bitcoin and everything seems confusing to me.  

I have a requirement where I need to create wallets for every user in my application screen. So, he will transfer some Bitcoins from his personal account to the newly created account and then purchase the items present in my application.  

Now, I have come across a website blockchain.info where we can create wallets by using their API.  

For using that API, we need to run a local service and use the API.

My question is, if blockchain.info goes down, then do all my accounts go down?
Or, it doesn't matter if blockchain.info goes down, because the accounts are created on Main Net of Bitcoin?  

Can someone, explain me this?
You would be using blockchain.info's wallet. If they were to stop providing their wallet service, you may not be able to access your Bitcoin at all (depends on your backup situation) although the Bitcoin is still present on the blockchain.

If you have proper backups of your blockchain.info wallet (you have the mnemonic for your wallet), then you can restore it to another wallet software and be able to access your Bitcoin from there.