ISP stands for Internet Service Provider. It's a service that allows you to connect/acess to the internet.
DSL is a type of internet provider. It includes broadband internet, but can be unavailable in a few locations. It also does not require dialing up nor requires phone services. If you were to choose DSL, you would need a DSL modem.
A router is a device that allows you to connect to multiple computers and devices to just one internet connection. A router is not necessary for connecting to the internet.
WWW stands for World Wide Web. WWW is much different from the term 'Internet'. The World Wide Web is a network that is simulated, that is connected by links. The WWW is just apart of the internet.