Citrix (accurately Citrix XenApp) is an application virtualization product that allows users to connect to their applications from a wide range of computer environments.
More info: http://en.wikipedia.org/wiki/Citrix_XenApp
XenApp can host applications on central servers and allow users to interact with them remotely or stream and deliver them to user devices for local execution.
Big advantage of this product is that the client can connect to the application regardless of the type of the server or client operating system. (e.g. you can access to a Linux application on a Windows client and vice versa)