API stands for Application Programming Interface. In practical terms, it is a defined set of rules that allows one piece of software to request data from or send data to another piece of software. When your e-commerce platform automatically sends order information to your shipping software, that connection is an API integration. When a new subscriber in your email platform automatically appears in your CRM, that is an API integration. When a customer's payment in Stripe automatically creates an invoice in QuickBooks, that is an API integration. The integration eliminates the manual step of copying information from one system to the other.
You need to think about integrations at three specific moments. The first is when you are choosing software: before committing to any tool, check what integrations it supports. A tool that does not integrate with the other tools in your stack creates a manual data transfer requirement that will consume staff time every week indefinitely. The second moment is when you are doing something manually that a computer could do: if you or a team member copies information from one system to another on a regular schedule, an integration can almost certainly automate that task. The third moment is when data is inconsistent across your systems: if your email platform has different information than your CRM, which has different information than your billing system, integrations that keep them synchronized will solve the problem.
Zapier and Make (formerly Integromat) are the tools that make API integrations accessible without engineering resources. Both platforms connect thousands of apps through a visual workflow builder where you define triggers and actions: when X happens in App A, do Y in App B. A new customer added to Shopify can trigger the creation of a contact in HubSpot, the addition of that contact to a specific Mailchimp list, and the creation of an invoice in QuickBooks, all in one automated workflow. Most integrations that a year-one business needs can be built in Zapier or Make without writing a line of code.
Native integrations, built directly between two specific tools by their developers, are always preferable to Zapier-based integrations when they exist. A native integration between Shopify and Klaviyo, for example, syncs more data, more reliably, with less configuration overhead than a Zapier workflow connecting the same two tools. Before building a Zapier integration, check whether a native integration between the two tools you want to connect already exists. Most major software categories have extensive native integration libraries.
Custom API integrations, built by a developer to connect two tools that do not have a native integration or a Zapier connector, are a last resort. They require ongoing maintenance when either tool updates its API, a developer relationship to maintain them, and documentation so someone understands what the integration does when the original developer is unavailable. The cost of a custom integration is not just the development cost but the ongoing maintenance cost. Evaluate alternatives before commissioning custom development.
Thinking about integrations before you choose your tools is the most efficient approach, because switching a tool after you have built your workflows around its lack of integration is significantly more disruptive than choosing an integrated tool from the start.