What is a socket, in technical but understandable terms?

A socket is a communication endpoint that a program asks the operating system to create. The program can use it to send and receive data without managing all the low-level network details itself.

For example, a web server can listen on a socket for incoming connections. A browser connects to it, and the two programs use their respective sockets to exchange requests and responses. The operating system handles the networking underneath.

On Unix-like systems, a socket often feels a bit like an open file. A program gets a file descriptor and can read or write data through it. That does not mean the socket is a regular file containing stored data, though. The operating system passes the data between communication endpoints.

Some sockets connect programs on the same machine rather than across a network. A Unix domain socket can have a filesystem pathname that other programs use to find it, but that pathname is not a file full of messages.

The type of socket matters. TCP sockets provide a connected stream of bytes. UDP sockets send and receive separate datagrams, without establishing a TCP-style connection. An IP address and port help identify network endpoints, but they are not the socket object itself.