UDP is an "unreliable" transfer protocol, ie if a packet is dropped or lost, it doesn't care. This is for high-demand but low-expectation connections (think streaming video: you don't care if you miss a frame or two because you can still see the entire video).
TCP is a reliable transfer protocol, with error correction. This method inherently is slower than udp as it needs to guarantee that every packet it sent successfully, thus it's important to use when you need high-fidelity transfers (ie, file transfers).
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