Online algorithms are algorithms designed to process data in real-time as it arrives, rather than in a batch processing manner. These algorithms are typically used in scenarios where data is continuously generated and needs to be processed quickly and efficiently. Online algorithms are commonly used in areas such as machine learning, networking, and finance. These algorithms are often optimized for speed, memory usage, and scalability, as they need to handle large amounts of data efficiently. Some common online algorithms include online learning algorithms, data stream algorithms, and online optimization algorithms.