Lucid keys are authored by experts on the group of organisms to which the key applies. The key comprises a list of characters and of taxa, and a database of information on the characteristics of the taxa. To make an identification, you will select character states that match your unidentified specimen: in effect, you will be describing your organism to the Player. The Player will then query its underlying database to find all taxa that match the selected characters. Successive selection of character states should narrow the list of possible taxa until only one or a few taxa remain.
Once a name has been provided by the key, the Player can branch to find information about the taxon such as descriptive notes, illustrations, sound recordings or videos. This information will have been provided by the builder of the key, or by a third party. A wide range of information may be available to you once you have a name for your organism. Of course, as well as the information provided within the key, the name you have found allows you to extract information from outside sources as well, since the whole field of biology stores information using taxon names as markers.
Further information on keys is available from the main KEYS section on the Lucidcentral site.