Socket Programming Basics
At a high level, a socket represents one endpoint of a communications channel. Network-based sockets transmit data across the wire, perhaps between two machines. Some operating systems support socket operations via the filesystem using a special file type. Streams-oriented, TCP socket communications involve steady connections between the two socket endpoints. Datagram-oriented, UDP connections pass data in minute packets, the order and delivery of which may be unreliable.
Typically, one socket represents the server side and the other is the client side. (I prefer the term "service" to "server," since the latter also refers to the machine on which the service runs.) The server listens for a client connection, exchanges some data with the client, and the two disconnect.
In code, the notion of a socket differs by the implementation. Java and ACE wrap it in an object. Traditional UNIX sockets are a combination of a file descriptor, represented by a numeric primitive, and a data structure that contains network information, such as IP addresses.
Merely creating a socket variable doesn't do anything. You have to tell it to wait for a connection (server-side) or initiate a connection to something (client-side). To wait for a connection, you first bind the socket to an address and port. Only one entity may listen on a given address:port pair at a time; binding is a way an entity stakes its claim. To describe networking in restaurant terms, for example, binding is the equivalent of renting out a spacenothing's there yet, but soon will be.
A bound socket is simply held so no one else can use it. To be available for connections, you must then listen on the socket. This means opening up your restaurant's doors for diners.
Finally, you accept client connections to perform a data exchange. Accepting clients means your host or hostess is waiting by the restaurant's door to admit and seat clients. For a blocking accept call, your host does nothing else until a client arrives. In a nonblocking accept call, the host periodically checks for clients and tends to other matters.