The emergence of autonomous AI agents represents a transformative leap in the evolution of artificial intelligence. These intelligent systems, capable of independently perceiving environments, making decisions, learning from experience, and executing multi-step actions without continuous human oversight, are redefining the boundaries of what machines can accomplish. Unlike traditional rule-based or supervised AI systems, autonomous agents integrate deep learning, reinforcement learning, natural language processing, and multi-modal decision frameworks to solve complex, dynamic, and often ambiguous real-world problems. This paper explores the technological underpinnings, capabilities, applications, and implications of autonomous AI agents. It critically examines their deployment in sectors such as healthcare, finance, cybersecurity, logistics, manufacturing, education, and scientific research. Furthermore, it addresses the ethical, legal, and socio-technical challenges arising from the increasing autonomy of machines, offering a roadmap for responsible innovation. Ultimately, autonomous AI agents are not merely tools—they are collaborators in a new era of intelligent automation.