An algorithm is a step-by-step plan for getting something done. Recipes, directions, and sorting laundry all have algorithm-like structure, but computer scientists make these plans exact enough for machines. Good algorithms can make the difference between a task taking seconds, hours, or longer than a lifetime.

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Every photo, song, message, website, and video game world must be represented as data. Computer science asks how real things can be turned into bits, symbols, files, and structures that machines can store and manipulate. This idea powers everything from emojis and streaming music to medical scans and space images.

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Modern software is built from layers: code, operating systems, networks, databases, chips, and user interfaces working together. When you tap a button, a request may travel through your phone, across the internet, into a data center, and back in a fraction of a second. Computer science helps make these invisible systems fast, dependable, and secure.

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Computer science connects to many kinds of work and communities: software development, cybersecurity, data science, robotics, game development, web design, research, accessibility technology, civic tech, and open-source projects. People in the field build tools, test systems, protect information, study data, design interactions, and use computing to help other fields solve hard problems.

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Review this chapter with practice based on your mistakes.