UDP has a considerably smaller header than TCP. UDP doesn't keep track of segmentation as TCP does, so packets are received and processed on a first come, first served basis. This is acceptable to some applications but not to others, and it comes with the benefit of much-reduced overheads, as well as quicker processing. There is no requirement for UDP to wait for a data stream to be completed before sending the next message, and so there is no need for the many flags to control traffic flow. The UDP datagram header is just 8 bytes long, compared to the 24 byte TCP header. This makes for very efficient data transfer (maximizing the amount of useful data being transmitted per datagram). This comes at the cost of data verification, guarantees of delivery, or datagram sequencing:

The UDP header contains only the Source Port and Destination Port, Length, and Checksum, which makes for a header less than half the size of TCP.