Ultra-Reliable Low-Latency Communication (URLLC) is a key enabler for many next-generation wireless network applications, including autonomous vehicles, industrial automation, and telemedicine. It is designed to meet stringent requirements of latency as low as one millisecond and reliability levels exceeding 99.999%. These characteristics make URLLC essential for mission-critical applications where delays or failures could result in severe consequences. This paper provides a comprehensive review of URLLC research, categorizing existing studies based on various URLLC applications. Additionally, we identify key challenges, including resource allocation, interference management, and the integration of emerging technologies like AI and edge computing. Open research problems of future wireless networks are discussed, providing potential directions for advancing URLLC capabilities. This paper serves as a detailed reference for researchers by highlighting state-of-the-art methodologies and emerging directions in URLLC.