Black's Law Dictionary defines Letters Rogatory as "A request by one court of another court in an independent jurisdiction that a witness be examined upon interrogatories sent with the request." It took me a minute, but I think that's clear.
The U.S. State Department posts instructions on how to prepare Letters Rogatory and how that information applies in individual countries.
The Inter-American Convention on Letters Rogatory governs the use of Letters Rogatory between the U.S., Spain and most countries in Central and South America. The Convention and a list of signatories is posted by the Organization of American States and Juris International.
Most countries that signed the Inter-American Convention have also signed the Additional Protocol to the Inter-American Convention on Letters Rogatory (OAS edition; Juris International edition).