An Application Delivery Controller (ADC) is a network device that is designed to improve the availability, security, and performance of applications. It is used to manage and control access to applications and can be configured to perform a variety of tasks, such as load balancing, content switching, and SSL offloading. 

ADCs are typically deployed in front of servers that host applications and are used to distribute incoming requests among the servers. This helps to improve the availability and performance of the applications, as it allows them to handle more requests and reduces the risk of a single server becoming overloaded. 

ADCs can also be configured to perform other tasks, such as caching content, optimizing traffic, and enforcing security policies. They are commonly used in enterprise environments to improve the performance and availability of critical applications, and can be deployed as hardware appliances, virtual appliances, or cloud-based services.