What Is An I.S.P (Internet Service Provider)?

To be simple, its someone who gives us internet. :)

There are many ways by which they enable us to connect to the internet. Well, you might know some, like dial-up, broadband etc.

From wiki :

An Internet service provider (ISP), also sometimes referred to as an Internet access provider (IAP), is a company that offers its customers access to the Internet. The ISP connects to its customers using a data transmission technology appropriate for delivering Internet Protocol datagrams, such as dial-up, DSL, cable modem, wireless or dedicated high-speed interconnects.
ISPs may provide Internet e-mail
accounts to users which allow them to communicate with one another by
sending and receiving electronic messages through their ISP's servers. (As part of their e-mail service, ISPs usually offer the user an e-mail client software package, developed either internally or through an outside contract arrangement) ISPs may provide other services such as remotely storing data files on behalf of their customers, as well as other services unique to each particular ISP.

An ISP is the company that gets you connected to the Internet.  You will typically connect to them through your phone line or cable, using a router (ie, a DSL router or a cable modem) which communicates between your PC and the ISP.