Planet of the Apes was a little known novel by Pierre Boulle published in 1963. Boulle is better known for Bridge over the River Kwai which also got the movie treatment. Boulle wasn't exactly fond of his own work and was surprised when it got picked up for a movie. The script wasn't too great either and went through several rewrites the final version getting a face-lift by Rod Serling of Twilight Zone fame. When Boulle watched the movie he said, "I wish I would have thought of that ending!"
The Book is about a couple space cruising and finding a message in a bottle, yes in space, and the story within is that of a scientist that travels to another planet which is populated by an advance civilization of Apes and humans are not the brightest animals of the bunch.
In the book the Apes' society is technologically advanced with Cars, planes etc. The movie didn't have the budget to match that so they decided to make it a primitive society and the rest is movie history.
Hollywood being the money-whore that it is, after the movie was a big hit decided to do a sequel. Beneath the Planet of the Apes was the answer and Charleston Heston was so against the idea of a sequel he only wanted to be on screen for like 15 min. That is why you don't see him until the end. In an interview Heston stated, "I had the idea of blowing up the atom bomb that destroys the planet... no planet... no sequels." But he didn't count on the cleverness of producers as the series spawned three more movies. The third movie and easily the weakest of the five has the two main Apes, Cornelius and Zira, going BACK to present day Earth. They got their because at some point the Ape scientist had built a spacecraft like the one Heston and his crew made it to the future and escaped the planet before the nuke destroys it. Zira and Cornelius are asked how they evolved, they explain that a virus kills all cats and dogs and humans start using primates as their pets. Apes, being smarter than cats and dogs, are also used as housekeepers and to perform other menial tasks and soon evolve. The humans freak out and decide to kill the two Apes, which they do, but not before Zira gives birth to... Caesar.
In Conquest of the Planet of the Apes Caesar has grown up and is under the care of Ricardo Montealban's character who is a circus owner. He has been hiding Caesar, but soon gets found out and Caesar is taken into custody. It is there where he rallies all the other Apes and war between Ape and Man breaks out... fucking epic. Seriously awesome movie.
The fifth movie, Battle for the Planet of the Apes, ties all the lose ends. After the War between men and apes the city suffered a nuclear meltdown and people are starting to feel the effects of the radiation (their skin melting... hence the mask wearing people from the second movie), and the Apes have to make one final stand against the radical humans.