The Den Den no Mi, is a Paramecia type Devil Fruit that allows the user to detect, communicate with and intersect the eletric wave of Den Den Mushi or in other words allow him or her to "hear" (Without words, telepathically) communication between Den Den Mushi and and people that using them.
The user can communicate with a specific Den Den Mushi without even needing to use a Den Den Mushi. He/She is also capable of intercepting calls between two people, much like Black Den Den Mushi. He/She also can watch what the Surveillance Den Den Mushi watches and broadcast what he/she saw with the Visual Den Den Mushi.He/She can also understand the conversation between Den Den Mushi.
This ability of gathering information makes the user an ultimate spy. Even the World Government is wasting time to search for the Devil Fruit and any information about it has been classified as "Top Secret".
The major weakness of this fruit is that the user can "wiretap" multiple communication lines but to decrypt the information(s) is up to the user's ability. After that, the user also might suffer a headache. The user still has standard Devil Fruit weaknesses