UDP is a connectionless, unreliable Layer 4 protocol.
Unreliable in this sense means that the protocol
does not ensure that every packet will reach its
destination. UDP is used for applications that provide
their own error recovery process or when
retransmission does not make sense. UDP is simple
and efficient, trading reliability for speed.
Why not resend? It may not be obvious why you
would not resend dropped packets if you had the
option to do so. However, real-time applications
such as voice and video could be disrupted by
receiving old packets out of order. For example,
suppose a packet containing a portion of speech is
received 2 seconds later than the rest of the conversation.
Playing the sound out into the earpiece
probably will sound like poor audio quality to the
user, because the user is listening further into the
conversation. In these cases, the application usually
can conceal the dropped packets from the end
user so long as they account for a small percentage
of the total.