Everything is bytes.  The problem comes when you want to make it into
something you recognize.  So if you can turn in into bytes and turn it
back you can use Datagrams.

In my experience, games and very simple network protocols.

Everything is bytes, including TCP.  But generally yes I think, TCP is
preferred for more complicated stuff, just because the Transport is
useful, and would have to be re-invented on datagrams.

Anything where you don't care if some data doesn't get to the other end or
arives in the wrong order.  Some reasons why you might not care:
   Because the data just isn't that important;
       (e.g. some logging).
   Because arriving late is just as bad as not arriving at all;
       (e.g. audio streaming).
   Because a higher-level of the protocol provides its own checking;
       (arguably DNS lookups fit this scenario).

No, not unless you convert the "complicated strings" to bytes yourself.  Same
goes for TCP/IP transmission (and, come to that, to files too).

   -- chris