You've probably never asked the question, "What is an Apple UDID?" This acronym stands for Unique Device IDentifier, and every iOS device has one. The UDID itself is a 40-character string including numbers and letters that is unique to your iPhone hardware. Apple sets standards for how the UDID should be used by application developers to protect user privacy.
A new study by Bucknell University security researcher Eric Smith has found that third-party app developers may not be following the rules. Some apps send private data such as name and location data from your iPhone in plain text along with the UDID. Some secure the transmissions with SSL, however this also means that the data being sent to external servers is unknown.