What is TCP/IP?

TCP/IP, Transmission Control Protocol/Internet Protocol, is a suite of communications protocols used to interconnect network devices on the Internet. TCP/IP implements layers of protocol stacks, and each layer provides a well-defined network services to the upper layer protocol. TCP and IP are the two protocols used by TCP/IP, as well as the (higher) application, (lower) data link and (lower) physical layer protocols.
 

Layer Protocols
5. Application DNS, FTP, HTTP, IMAP, POP3, SMTP, SSH, Telnet, SSL, …
4. Transport TCP, UDP, …
3. Network IP (IPv4, IPv6), ICMP, ARP, …
2. Data Link 802.3 (Ethernet), 802.11 (Wi-Fi), PPP, …
1. Physical Ethernet (NIC), Wireless (NIC), Cat 5/RJ-45, …