The Sarlacc pit, also known as the Great Pit of Carkoon, is the centerpiece of one of the most iconic scenes in the original Star Wars trilogy. Luke Skywalker, captured in an attempt to rescue Han Solo from Jabba the Hutt, has been sentenced to die by "walking the plank" from a hovering skiff into a giant hole in the planet Tatooine's desert sands — one occupied by an enormous but immobile beast known as the Sarlacc monster.
This pivotal scene in Return of the Jedi serves to display Luke's new mastery of The Force — the mystical universal energy that gives a Jedi Knight his power. In A New Hope, Luke was a backwater (can you be backwater on a desert planet?) farmboy given some basic training by the long past-his-prime Ben Kenobi. In The Empire Strikes Back, Luke advanced his training with the Jedi Master Yoda, learning new techniques such as levitation and precognition. By the time of Return of the Jedi, Luke has completed his training and is now a full-fledged Jedi Knight, the last one in the galaxy. Finally, the audience gets to see what a real Jedi can do, and Luke's display of superhuman acrobatics and ability to block blaster bolts with his lightsaber prove more than a match for Jabba's entire gang, including the fan-favorite bounty hunter Boba Fett. Ultimately, it's a set-up for Luke's final showdown with Darth Vader, establishing him as a credible challenge for the Sith Lord.
The Sarlacc pit itself is similar in form and function to a giant ant lion trap, a conical pit in the loose, shifting sands that offers no purchase a victim can use to try to climb back out. Struggling just makes one slide faster down into the center. At the center of the pit is the cavernous maw of the Sarlacc monster, easily 10 feet in diameter and ringed with a number of lashing tentacles it uses to expedite a victim's descent. In the Special Edition, George Lucas made the decision to add an enormous CGI beak (which I hate) to the Sarlacc, which extended from the center like a second mouth, in addition to the puppet tentacles. The mouth is also ringed with several rows of fang-like teeth which serve no obvious purpose. If I may be so bold to add my own ideas to the mythology, they may be left over from the Sarlacc's youth when it was too small to simply swallow victims whole, or perhaps to bite and hold the limbs of larger prey such as the Krayt dragon which might otherwise be capable of escaping.
According to Jabba the Hutt, as translated by C-3P0, victims of the Sarlacc are slowly digested over a period of 1,000 years in what must be agonizing torture. While there was no reason to believe the victim would actually survive more than a few days of this, the Expanded Universe added a ridiculous, thermodynamics-confounding explanation that the Sarlacc actually keeps its victim alive for the duration by pumping nutrients back into it, in a manner I can only assume is similar to the Matrix liquefying the dead to feed to the living. In either case, the immobile Sarlacc doubtlessly has a slow, efficient digestion process due to its manner of feeding, relying on inattentive desert-dwelling creatures to accidentally fall into its mouth.
In the movie, the Sarlacc's insatiable gullet was the final ignoble resting place of Boba Fett (accidentally knocked over by a blind Han Solo), along with most of Jabba's minions (knocked off of the hovering skiffs by Skywalker in his bid for freedom) and quite nearly Lando Calrissian (rescued from a tentacle's grip by Han and Chewie). In the Expanded Universe, Boba Fett was able to escape from the Sarlacc, being the only individual in the galaxy known to have done so. In fact, Black Pawn tells me he managed to do it twice, after somehow falling in a second time sometime after the Battle of Endor.