What is java.net.Socket…?

In this paragraph what is java.net.Socket, java.net.DatagramSocket, and java.net.ServerSocket?
The value for SO_TIMEOUT is set by using the setSoTimeout() method, supported by java.net.Socket, java.net.DatagramSocket, and java.net.ServerSocket. ThesetSoTimeout method accepts an int, which is the amount of time in milliseconds allowed before a timeout is encountered (for example, a value of 2000 equals two seconds).