Though few but the most devoted computer science students are even aware of the existence of application product interfaces (or APIs), they serve an essential function in every piece of tech you use.

APIs are, in short, an agreement between two programs to work with one another and share data (under certain restrictions) and can be as limited or as open in scope as the developers desire. Without APIs, none of your technology would work as fluidly, as they enable the services we use every day to interact with one another to create a seamless user experience. Without APIs, you would have to use each application individually on your phone or computer, switching back and forth between different programs instead of being able to transition from one service to another, with the two sharing data.

Whether you’re reading this as a comp sci student trying to learn more about the inner workings of your technology, an amateur application developer looking to patch your product into an existing framework of programming, or a business looking to expand the potential utility of an existing product, this crash course on APIs aims to provide you with a general knowledge of the most useful APIs available to the public. While not all of the APIs in the categories below may be open source, most of them are reasonably priced and easy to work with. Take a look at options for each category to determine which API developer will be right for you and which API in each category will add the most value possible to your product.

Without further ado, let’s begin.

Weather APIs: A Hurricane of Information

Take a look at your smartphone, see for a moment if it has a weather widget with information from a source like the Weather Channel. That is, in fact, a weather API in action.

Weather APIs from sources like Visual Crossing allow you to keep track of real-time weather information in your area, access a massive back catalog of weather data, and inform users of weather alerts and critical conditions within moments of them being reported. Users may find that having access to a wealth of weather data is useful for a variety of purposes: students can use the historical data for projects, vacation planners can get a solid idea of the typical climate of another area, and users across demographics will appreciate getting weather updates sub-hourly. This versatility is also why weather APIs are among the most common APIs you can find on most modern gadgets.

Other similarly useful APIs include Census APIs, as well as any other kind of API that patches into a database of existing information to provide a service.

Social Media APIs: Patch into Social Networks

Another popular series of APIs that are more often open source is social media APIs, or APIs that allow an application to work with social media sites like Facebook, Twitter, and Instagram. These APIs allow users to log into any number of sites or applications using their social media profiles, limiting the number of accounts and passwords they have to create to access old and new services and enabling them to share content across platforms with a single click.

You’ve likely already used this type of API in your day-to-day life as well, logging into a site using your Facebook or Instagram account: its unsurprisingly common to see this kind of API being integrated into a variety of applications, as consumers have come to expect that level of convenience in older and newer apps.

Marketplace APIs: All Your Financials in One Place

Similarly to social media APIs, marketplace APIs allow you to log into commercial sites with one account, as well as giving you the ability to post items for sale across a variety of commercial sites at once, direct users to new goods, and more. If you want to lead your customers down a rabbithole of related products, making the absolute most out of their investment in your application and trying to secure purchases for the future, you may want to obtain a marketplace API.

APIs have run the world silently, working in the background of all of our favorite applications for years now: you’ve definitely used a few, and have come to appreciate and expect the kind of conveniences they offer in your day-to-day life. Whether you’re an app developer, a company executive, or a curious student, looking into APIs can reveal how much is done behind the scenes of your most-visited sites and applications and open doors of opportunity that can drown you in cash.