A rootkit is a type of malicious software designed to gain unauthorized access to a computer system while hiding its presence. It allows attackers to control the system remotely, often without the user's knowledge. Rootkits can modify the operating system and other software to conceal their activities, making them difficult to detect. Rootkits can be installed through various means, such as exploiting vulnerabilities in software or tricking users into downloading them. Once installed, they can enable other types of malware, steal sensitive information, or create backdoors for future access. Detecting and removing rootkits often requires specialized tools and techniques.