Got a job for you, Marked One...
PDA descriptions are a feature of all three S.T.A.L.K.E.R. games that play a supporting role during gameplay.
The player's PDA is his guide to the game world; it contains maps, statistics of NPC's, and the various missions that he is keeping track of. The PDA also offers descriptions of these tasks: if the player clicks on the "show text" button next to a mission, it will show a brief summary of information pertaining to the task currently selected. These are sometimes helpful if the player cannot remember what he is supposed to be doing at the moment.
In the map feature of the PDA, the player can access the locations of stashes he's picked up either as the result of completing a mission for someone or, more frequently, from the PDAs of dead NPCs. By hovering the mouse cursor over the purple or green circle with the triangle inside, he can see a description of the stash, though these are usually more annoying than helpful.
The PDA can also be informative for first-time players; as the character experiences things within the game, the encyclopedia is auto-updated, and can provide info on artifacts, factions, mutants, etc. It also contains a small archive for local tall-tales that the character picks up, and though they aren't helpful, they are sometimes rather entertaining to read.
A fairly useless feature, the player's stats are also present - in Shadow of Chernobyl it gives an all-inclusive list of how many artifacts the player has won, how many and what type of mutants he's killed, and how many and what affiliation of other stalkers he's killed, as well as a total. In Call of Pripyat the statistics show the most dangerous mutant killed, the weapon that the player uses most often, etc.