New to shopify api, not understanding "apps"

[mtek_dan123]

I'm sure my question will sound unintelligent but I'm asking it anyway in the hopes to get clarification. I work for a small fulfillment company. One of our clients has a shopify store. They want us to pull their order data nightly to log into our system, and then post tracking info back the next day. We don't do much with APIs here but we have in the past. They're all simple, we have the api credentials, and use curl calls. But reading shopify documentation I'm confused when it's talking about "apps". And needing to create an app with a url and redirect url. I don't know why we would need to create an app. We wouldn't have an "app url". Or a redirect url. It would solely be a connection from our system to theirs, right? Why would we need oAuth when there's only 2 parties? We would just want their api creds to be able to make the calls in our (old) coding language. Am I totally misunderstanding how this works? Thanks for any help.  

[mtek_dan123]

I found out my answer is to make a "private app". Then we do not have to use oAuth or any url or url redirects from our end. It simply gives us the API creds that we use to make the calls.