1. A system comprising: one or more processors; one or more computer-readable media; computer-readable instructions on the one or more computer-readable media which, when executed by said one or more processors, cause said one or more processors to implement a method comprising: describing one or more software extensions using descriptions, the extensions being configured for incorporation in a software platform executing on a client; and delivering the descriptions of the one or more extensions to the client via a network, the descriptions being configured for use in downloading the software extensions via the network; said acts of describing and delivering being configured to enable software to be delivered over the network.