A/B testing is an experimental method that compares two versions of a webpage, email, advertisement, or content to determine which generates better results. It is used to optimize conversion rates, improve user experience, and make data-driven decisions. Tools like Google Optimize, Optimizely, and Unbounce allow for real-time testing and analysis of user behavior. It is a key technique in digital marketing to maximize the impact of advertising campaigns and improve the effectiveness of acquisition and retention strategies.