套接字是一种数据I/O机制。端口是通信协议的契约概念。套接字可以没有端口而存在。一个端口可以没有特定的套接字而存在(例如，如果同一个端口上有几个活动的套接字，这可能是某些协议允许的)。

端口用于确定接收端应该将数据包路由到哪个套接字，在许多协议中，但它并不总是必需的，接收套接字的选择可以通过其他方式完成——端口完全是网络子系统中协议处理程序使用的工具。例如，如果协议不使用端口，数据包可以到所有侦听套接字或任何套接字。

A socket is a communication endpoint. A socket is not directly related to the TCP/IP protocol family, it can be used with any protocol your system supports. The C socket API expects you to first get a blank socket object from the system that you can then either bind to a local socket address (to directly retrieve incoming traffic for connection-less protocols or to accept incoming connection requests for connection-oriented protocols) or that you can connect to a remote socket address (for either kind of protocol). You can even do both if you want to control both, the local socket address a socket is bound to and the remote socket address a socket is connected to. For connection-less protocols connecting a socket is even optional but if you don't do that, you'll have to also pass the destination address with every packet you want to send over the socket as how else would the socket know where to send this data to? Advantage is that you can use a single socket to send packets to different socket addresses. Once you have your socket configured and maybe even connected, consider it to be a bi-directional communication pipe. You can use it to pass data to some destination and some destination can use it to pass data back to you. What you write to a socket is send out and what has been received is available for reading.

Ports on the other hand are something that only certain protocols of the TCP/IP protocol stack have. TCP and UDP packets have ports. A port is just a simple number. The combination of source port and destination port identify a communication channel between two hosts. E.g. you may have a server that shall be both, a simple HTTP server and a simple FTP server. If now a packet arrives for the address of that server, how would it know if that is a packet for the HTTP or the FTP server? Well, it will know so as the HTTP server will run on port 80 and the FTP server on port 21, so if the packet arrives with a destination port 80, it is for the HTTP server and not for the FTP server. Also the packet has a source port since without such a source port, a server could only have one connection to one IP address at a time. The source port makes it possible for a server to distinguish otherwise identical connections: they all have the same destination port, e.g. port 80, the same destination IP (the IP of the server), and the same source IP, as they all come from the same client, but as they have different source ports, the server can distinguish them from each other. And when the server sends back replies, it will do so to the port the request came from, that way the client can also distinguish different replies it receives from the same server.

套接字基本上是网络通信的端点，至少由一个ip地址和一个端口组成。在Java/ c#中，套接字是双向连接一侧的高级实现。

还有Java教程中的一个(非规范的)定义。

在阅读了这些优秀的向上投票的答案后，我发现对于我这个网络编程新手来说，有以下几点需要强调:

TCP-IP连接是连接一个地址:端口组合和另一个地址:端口组合的双向路径。因此，每当您打开从本地计算机到远程服务器上的端口的连接(例如www.google.com:80)时，您也将计算机上的一个新端口号与该连接关联起来，以允许服务器将内容发回给您(例如127.0.0.1:65234)。使用netstat查看你机器的连接是很有帮助的:

> netstat -nWp tcp (on OS X)
Active Internet connections
Proto Recv-Q Send-Q  Local Address          Foreign Address        (state)    
tcp4       0      0  192.168.0.6.49871      17.172.232.57.5223     ESTABLISHED
...

它们是来自两个不同领域的术语:“端口”是TCP/IP网络的概念，“套接字”是API(编程)的东西。“套接字”是通过将端口和主机名或网络适配器组合成一个数据结构(可以用来发送或接收数据)而形成的(在代码中)。

端口是最简单的部分，它只是套接字的唯一标识符。套接字是进程可以用来建立连接和相互通信的东西。高个子杰夫有一个很好的电话比喻，但并不完美，所以我决定修正它:

IP和端口~电话号码 插座~电话设备 连接~电话 建立连接~拨打号码 流程，远程应用程序~人员 留言~语音