For things like P2P UDP is a much better fit, because TCP sends an ACK (ACKnowledgement) to the sender for every received packet.
Since UDP is unreliable no such ACKs will be send and it's up to the programmer to detect packet loss. The latency is therefore with UDP lower (and hence throughput higher), because the provider can push packets without waiting for a response.
For instance in P2P the receiver only needs to send a NAK (Negative AcKnowledgment) if and only if a packet/part is missing and then it will be resend by the provider.
TCP sends an ACK (ACKnowledgement) to the sender for every received packet.
Not quite. TCP receivers only need to send ACKs when they have received enough data to fill the sliding window. That may be every packet, every other packet (the recommended by the RFC) or more.
