In synchronous code, each operation waits for the previous one to finish before running; in asynchronous code, operations can start and the program keeps going without waiting for them to complete.
Synchronous execution runs tasks strictly one after another, in order — the program is blocked, doing nothing else, while a slow operation (like a network request) completes.
Asynchronous programming lets a program start a slow task (like fetching data from a server or reading a large file) and continue doing other work while it waits, instead of freezing. When the slow task finishes, the program is notified and handles the result — often through callbacks, promises, or async/await syntax in languages like JavaScript and Python.
A web page loading data from an API asynchronously stays responsive — the user can still scroll and click while the data loads in the background. If that same request were synchronous, the entire page would freeze until the server responded.
Last reviewed: September 2026