An app is short for application, which is a software program. Apps can help people play games, read the news or find their way. Most of us never think about what an app is made of, or about where its words and pictures come from when we open it. That is the question here. How does a program on a phone get its work done, and who decides what it may do? Let's take it one part at a time, and look at some of the rules and some of the words people use, so that each term is clear before we move on, and you know what it means when you meet it.

Here is some history. The idea of a program stored inside a computer's memory was introduced in the late 1940s. An operating system manages resources such as memory, the processor and devices for the programs running on a computer. Keep that date in mind as you read on, and let's follow the thread from there, one piece at a time, to see where it leads, and how each piece connects to the next one in the chain.

In a client-server system, the client sends a request and the server sends back a response. A web server sends a website's content to the device that asks for it. A web server answers with a status code, such as 200 for success and 404 when a file cannot be found. Each of those words has a plain meaning, and each one shows up again and again whenever a program talks to a computer far away. Learn them once, and you can spot them anywhere. Let's see how they fit together.

Servers come in two kinds. A static web server sends its files as they are, and a dynamic one builds pages with an application server and a database first. A database is a repository of information that a server can pull content from to build a page or screen. A browser may make several requests for one page, to fetch the page's other files such as JavaScript and CSS. Picture a busy day for a server, with many requests coming in, one after another, from many different places at the same moment.

A progressive web app is a website that can be installed on a device and launched like any other app. Some apps, such as progressive web apps, are built to keep working on a slow connection or when offline. JavaScript is a scripting language that makes web content update on its own, and it is the third layer after HTML and CSS. In web code, an event such as a click can run code, for example code that changes a label.

Android handles its apps in a particular way. An Android app is packaged in an APK file, and Android devices use that file to install the app. An Android app, by default, runs in its own process, and the system starts that process when one of the app's parts needs to run. The Android system follows the principle of least privilege: each app, by default, can reach only the components it needs. It is a good idea to learn these words, since they show up again whenever people talk about how a phone is run.

Who builds all this? Apps are made by developers, who may work alone or in a team. Computer programs are written in programming languages such as C, Python and C++. A program is first written in a computer language by a person and then translated into a form the computer can run. Developers use special tools to build and fix apps, such as Android Studio and Xcode. As of November 2026, Apple lets developers test early versions with up to 10,000 outside testers through TestFlight. As of November 2026, Apple reviews apps before they appear on its App Store, and says that apps that cheat the review system can be removed and the developer expelled from its developer program.

Apple opened its App Store in 2008. More than 100 million apps were downloaded in the first 60 days after that, and by January 2010 more than three billion had been downloaded. As of November 2026, Apple handles payments and hosting for apps in its App Store, so developers do not have to build their own store. As of November 2026, developers can earn money from an app by charging for the download or for features inside it. Some apps offer auto-renewable subscriptions, which the App Store has supported since 2011. The Computer History Museum says the app store turned the phone into an open-ended, multifunction device, and Apple says the App Store has transformed how people live, work and play.

What an app may see is a choice. Apps may ask for permission to use information on a device, such as contacts, calendar entries and location. Many Android app permissions are for private user data, which includes location and contact information. A phone's settings let a person change an app's permissions after they first answered. Some apps share information from a phone with other companies that show ads based on a person's location and interests. As of November 2026, on iPhone a person can tell an app not to track them and still use the app's full features. As of November 2026, each app on Apple's App Store has a privacy label meant to help people understand what data the app collects and how it is used.

Staying safe means keeping up with updates and knowing the tricks. Updating an app gives you the latest features and makes it more secure, and many software updates are made to fix security risks. Malware is software made on purpose to cause harm. Phishing is when criminals try to get people to open harmful links or attachments, and it can come as an email, text, direct message or phone call.

Ask a grown-up before you get an app. Parents can set up a phone so that a child has to ask a grown-up before getting an app. Some phones have settings that let a parent make a child ask permission before downloading or buying apps. The COPPA rule was designed to protect children under age 13. As of November 2026, Apple's Ask to Buy is turned on by default for children under 13. When a child asks to get an app on an iPhone, the parent can approve or decline the request from the parent's own device.

Words to know

• client: asking program
• database: stored information
• process: running program
• phishing: tricking people
• malware: harmful software