The Shopify API lets you do almost anything the merchant can from their admin. In a nutshell: OAuth 2 + HTTP + (JSON | XML) over REST. Adapters for Ruby, Python, PHP, Java, and Node.js exist, but all you really need is HTTP and some love.
Dive into making your first Shopify app with our getting started guides and videos. We’ll walk you through the process of obtaining an API key, creating a test shop, playing around with the API, and how to use Shopify’s single sign-in.
Give us a shout on the Shopify API Forums if you run into any trouble.
Ready to publish your app? Just hit “Submit” in your Partner account. After your app is approved, you may get a featured spot at the top of the Shopify App Store. Shopify's Billing API makes it easy to bill customers, supporting recurring monthly charges, one-time charges, and combinations thereof.