An app can feel like a small world of its own, but it lives inside a bigger system on the phone. An app is short for application, which is a software program, and it can help people play games, read the news or find their way. Software is the programs a computer runs, and hardware is the equipment, while the operating system is the main software that manages a computer's resources and lets other programs run. Android and iOS are two operating systems for phones.

The iPhone first appeared in 2007, and Apple opened its App Store in 2008. In the first 60 days after that opening, more than 100 million apps were downloaded, and by January 2010 more than three billion had been downloaded, and apps are made by developers, who may work alone or in a team.

Every Android app lives in its own security sandbox, where each app is treated as a different user of the system. The Android system also follows the principle of least privilege, so by default each one can reach only the components it needs. You may have heard that an app can use anything on a phone. In fact, an app reaches only what it needs unless a person grants more.

Some things sit behind a permission. A program may ask for permission to use information on a device, such as contacts, calendar entries and location. It can ask before it uses the camera or contacts on a phone, and you can say Allow or Deny. Many Android permissions are for private user data, which includes location and contact information. A person can also choose how long an app may use location, camera or microphone, such as only while using the app or asking every time. A phone's settings let a person change those answers after they first gave them.

Here is where a server comes in. A server is a computer on a network that stores things like files and sends them to other devices, and in a client-server system the client sends a request and the server sends back a response.

An app can send a notification, a short message that shows up on a phone's screen. Android notifications are meant to give timely information and reminders even when the app is not in use. Updating an app gives you the latest features and makes it more secure, since many software updates are made to fix security risks, and a phone can be set to update its apps automatically.

As of November 2026, Apple reviews apps before they appear on its App Store. Apple handles payments and hosting there, so developers do not have to build their own store. It also says that apps that cheat the review system can be removed and the developer expelled from its developer program. As of November 2026, developers can earn money from an app by charging for the download or for features inside it. Some apps and some things inside apps cost real money, and in-app purchases let a person pay for extra things inside an app.

Some apps share information from a phone with other companies that show ads based on a person's location and interests, and as of November 2026, Apple's App Tracking prompt lets a person tell an app not to track them, and then the app cannot get the system advertising identifier. You may have heard that saying no to tracking makes an app stop working. In fact, as of November 2026, on iPhone a person can tell it not to track them and still use its full features.

Apps for children have their own rules. As of November 2026, Apple's guidelines say Kids Category apps must not have links out of the app or ways to buy things unless they are in an area behind a parental gate. Apps and games can have an age rating, and parental controls can block games and apps by their rating. The COPPA rule was designed to protect children under age 13, and it applies to mobile apps that connect to the Internet.

Before a child gets an app, a grown-up should be asked. 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. As of November 2026, Apple's Ask to Buy is turned on by default for children under 13. As of November 2026, 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. Parents can also block or allow particular apps on a child's phone. A parent can set daily time limits on a child's phone, and Google's Family Link can also set a bedtime. On Apple and Google phones, a parent or guardian creates the account for a child under 13.

Accessibility features in apps help all users. For example, voice commands can replace touch gestures while someone cooks. According to the World Bank, 15 percent of the world's population has some type of disability, and people with disabilities depend on apps that support accessibility. That figure is as of November 2026.

Words to know

• sandbox: app's own space
• server: network computer
• notification: short message