Tatooine is the most important planet in both the original and prequels films. Tatooine is a desert planet that features in every film except Star Wars V - The Empire Strikes Back and Star Wars III - Revenge of the Sith . That is a bit of a lie, in Revenge of the Sith, we see Luke Skywalker being given to his adopted parents.
Tatooine is the birth place of Anakin Skywalker who was born to a female slave, Shmi Skywalker. Tatooine has no redeeming features, the Empire has no interest in the planet, only visiting when they want something like the droids, R2-D2 and C-3PO who escape from the clutches of the empire to the planet. Tatooine is controlled in large parts by scum and villainy such as the Hutts, Jabba, the Hutt in particular.
Once Emperor Palpatine has gained control of the galaxy, Anakins children, Luke Skywalker and Princess Leia Organa are separated. Whilst Princess Leia Organa lives in near perfection, Luke Skywalker lives on the barren planet with relatives. Tatooine is similar to Arrakis in the Dune universe.
Mos Eisley is the first place that we see in the first original movie, it is where Obi-Wan Kenobi takes Luke Skywalker and the droids to find a person to take them to Alderaan. It is described by Obi-Wan Kenobi as "Mos Eisley Spaceport. You will never find a more wretched hive of scum and villainy. We must be cautious."
In the cantina, they meet Han Solo who is willing to take them to Alderaan as he believes it will help pay off the money he owes Jabba, the Hutt. Later Han Solo meets Jabba outside his Millennium Falcon. The scene where he meets Jabba originally had a human but for the special edition, the slug like creation that Jabba is was superimposed.
Mos Espa is a village on Tatooine that was the birth and childhood place of Anakin Skywalker. When the craft carrying Padme Amidala is forced to land, the Jedi Knights of Qui-gon Jinn and Obi-Wan Kenobi land outside the village and come across Anakin Skywalker. The village is the birthplace and childhood home of Anakin Skywalker. His mother Shmi Skywalker alleges she became pregnant without the use of a man.
Jabba, the Hutt runs business from here. He controls the Pod-Racing business which in the first episode, he is seen as the starter for the race. Pod-Racing is one of the big businesses that occur in Mos Espa.
Gangster Jabba has a palace in the wastes of Tatooine. It is a location that the Rebel Alliance visit to rescue Han Solo who is now frozen in Carbonite on the Palace walls.
Tatooine will forever be associated with Tunisia from where the scenes in the first two trilogies were made. Mos Espa was filmed near the town of Nefta in the middle western area of the country, near Chott el-Jerid lake. Mos Eisley, the space port and the home area of Luke Skywalker was also filmed in this area. The country, the birthplace of the Arab Spring when people in a number of Arab countries rose up against their leaders was deemed too dangerous, the producers didn't return to Tunisia, instead opting for Dubai and a new planet called Jakku.
The home of Luke Skywalker lived in before he joined the rebelliion was once a hotel built long ago but when George Lucas came along, they change it Luke's homestead.1
Although most of Tatooine was filmed in Tunisia, there were some scenes filmed elsewhere, Death Valley National Park in California/Nevada provided for scenes where Luke Skywalker met his future mentor Obi-Wan Kenobi.
Mos Espa slave quarters were filmed in a real-location, the village Medenine in Tunisia was used for the quarters where Anakin Skywalked lived as a slave with his mum.
The Great Pit of Carckoon, where Luke Skywalker, Chewbacca and Han Solo were to be fed to the Sarlacc Pit was filmed in Yuma Desert, Arizona and not as you would have expected Tunisia.