TCP/IP one of the protocols used, but not for communication between the PCs (although a lot of people believe this). Sharing takes place using the NetBEUI (NetBIOS) protocol. Your computers communicate between each other using MAC addresses (your 4 port switch on your router routes information between the computers based on their mac addresses). Your IP address (the later part of TCP/IP) is used by the router portion of your router to route information two and from the destination that it is going off your local network (most SOHO routers do not, however, route information. Instead, they send it off to your ISP's router. This is more commonly know as gateway routing). The TCP, Transmission Control Protocol, establishes and regulates communication between the computers. It guarantees delivery of data, as well as guarantees that data will get to your computers in the order that they were transmitted. However, because segments require IP addresses in the packets, none of this could take place without an IP address.
To put it in other terms, TCP/IP will be used for sharing between your computers