RJ Zaworski explains DNS for web developers:
DNS can use a similar TCP/IP stack, but being parts of a simple system, most DNS operations can also travel the wire on the Internet’s favorite Roulette wheel: the User Datagram Protocol, UDP.
On a good day, UDP is fast, simple, and stripped bare of unnecessary niceties like delivery guarantees and congestion management. But a UDP message may also never be delivered, or it may be delivered twice. It may never get a response, which makes for fun client design–particularly coming from the relatively safe and well-adjusted world of HTTP. With TCP, you get an established connection and all kinds of accommodations when Things Inevitably Go Wrong. UDP? “Best effort” delivery. Which means a packet thrown over the fence with a prayer for a soft landing.
It’s a good read if you’re new to DNS.