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The only difference between recv() and read(2) is the presence of flags. With a zero flags argument, recv() is generally equivalent to read(2) (but see NOTES).
The send() call may be used only when the socket is in a connected state (so that the intended recipient is known). The only difference between send() and write() is the presence of flags. With zero flags parameter, send() is equivalent to write().
Unlike the recv() call, which can only be used on a connected stream socket or bound datagram socket, recvfrom() can be used to receive data on a socket whether or not it is connected. If no messages are available at the socket, the recvfrom() call waits for a message to arrive unless the socket is nonblocking.
Returned value. If successful, recv() returns the length of the message or datagram in bytes. The value 0 indicates the connection is closed. If unsuccessful, recv() returns -1 and sets errno to one of the following values: Error Code.
The difference is that recv()/send() work only on socket descriptors and let you specify certain options for the actual operation. Those functions are slightly more specialized (for instance, you can set a flag to ignore SIGPIPE, or to send out-of-band messages...).
Functions read()/write() are the universal file descriptor functions working on all descriptors.
Per the first hit on Google
read() is equivalent to recv() with a flags parameter of 0. Other values for the flags parameter change the behaviour of recv(). Similarly, write() is equivalent to send() with flags == 0.
read() and write() are more generic, they work with any file descriptor.
However, they won't work on Windows.
You can pass additional options to send() and recv(), so you may have to used them in some cases.
I just noticed recently that when I used write() on a socket in Windows, it almost works (the FD passed to write() isn't the same as the one passed to send(); I used _open_osfhandle() to get the FD to pass to write()). However, it didn't work when I tried to send binary data that included character 10. write() somewhere inserted character 13 before this. Changing it to send() with a flags parameter of 0 fixed that problem. read() could have the reverse problem if 13-10 are consecutive in the binary data, but I haven't tested it. But that appears to be another possible difference between send() and write().
Another thing on linux is:
send does not allow to operate on non-socket fd. Thus, for example to write on usb port, write is necessary.
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