(A UN*X signal:)
SIGPIPE is the "broken pipe" signal on UNIX, Linux, and the other Unixoids. It is sent to a process which tries to write to a pipe (or socket, or any similar stream) which the other side has already closed for reading. There is nothing which could be done with such data -- it must vanish.
TCP connections are modeled after UNIXy pipes, so it should come as no surprise that writing to a half-closed TCP connection will result in the OS delivering SIGPIPE.
Here's one way to generate SIGPIPE in the comfort of your own $HOME. We shall use a named pipe (or fifo)...
$ mkfifo foo
If we write to foo, the process will
block, waiting for a
reader (the data are still not lost!).
$ cat bar >foo &
Now
redirect a process which consumes no
input to "read" from foo:
$ echo < foo
Voila!
SIGPIPE!