Bitcoin neither has, nor requires, a central Internet server or authority. Users running the Bitcoin software communicate directly with each other over the Internet. Through this distributed network, the Bitcoin network protocol communicates both transactions – instantaneous messages transmitting a desire to send money, and the blockchain – the permanent record of accepted transactions.
The communication between clients, also called nodes, is called relaying. Bitcoin nodes have relaying rules – nodes inspect transactions and blocks, and won’t pass along messages that are spam-like (having less than the minimum required transaction fee or containing trivially small “dust” payments) or are invalid.