The Private Sniffer is similar to the Wireless Sniffer. However, if you were in a server, it would show only your wireless frequencies. The crafting recipes are also the same, however, you would use blaze rods instead of Obsidian Sticks. The Private Sniffer also notifies you if your frequency has been jammed.
It shows all of the 5,000 frequencies too, but the first 1000 are labeled "Public" and the next 4,000 are labeled "Shared". When you Right or Left click on a frequency square in "Shared" area you toggle the "Private" mode. When the block is green, only you can set the Wireless Remote or Wireless Transmitter to the "Private" frequency.
On SMP its advisable to use this tool to reserve some frequencies. Only that way other players cannot use the reserved frequencies or change reserved frequency names.