This is basically a dictionary which covers all the radicals and their variants; several components are included.. The data includes meaning, notes, and etymology. Frequency information is included for the KangXi radicals and their variants (this is RANK, higher means more frequent). There is also a note for each character whether there is any point in learning it as a standalone character. The additional radicals from the modern simplified chinese tables are covered where possible, though in some cases these radicals only exist in extension B.
Updated this for Pleco 2.0, the updates include:
Feedback is welcome, submit to this Plecoforums topic here: http://plecoforums.com/viewtopic.php?f=7&t=1149
In addition to abovementioned sources all of the dictionaries within Pleco were consulted before writing the definition, a couple verbatim copies came from Unihan or CEdict. I also found component information on a number of sites, I think one of the main resources I used was a variant of the Unihan database somewhere on mandarintools.com. Wenlin was consulted for some component information and some of the etymology updates
Note Pleco's default font does not include all of these characters, particularly those in Extension A. To display all the characters I've changed the font to Sun-ExtA. Sun-ExtA is freely available on the net, but takes a whopping 26megs of data… You only miss a handful of characters if you don't do this, so don't worry about it too much if you don't want to take that hit. As an added bonus Sun-ExtA is the most complete Unicode font I've found to date - if you're looking up Wikipedia articles on your handheld containing obscure languages odds are they'll render with Sun-ExtA.
Sun-ExtA is a free font and available from the following locations:
*Pleco 2.0 Font Support* 2.0 on windows mobile uses a font file called ZYSong.ttf which is stored in \Program Files\Pleco on the storage card. Back this file up somewhere else, rename the SunExtA font to ZYSong.ttf and place it in the same location. Note this isn't officially supported. I haven't had problems with this, but if you have any problems restore the original font file.