Let's start at the beginning... So what are they ?
Jawas are an alien species from the Star Wars universe. Ther're well known scavengers, droid traders and occupational chancers. Being a rather skittish race, they reisde in packs and are more likley to run away from a fight than start one. As inhabitants of the desert world of Tatooine, they live onboard massive tank-like vehicles called Sandcrawlers, which protects them from sand storms and attacks from other aliens like the Tusken Raiders.
Thought to be a species evolved from rodents, Jawas measure approximately 1.0 to 1.4 meters in height. Jawas are commonly dressed in long brown robes, with large hoods hiding everything but the glow of their eyes. Adorded with leather bandoliers and gun straps criss crossed over their shoulders, they normally brandish rudimentary Ion weapons or other types of equipment for restraining droids.
Laying out the foundations... the original conception
Long before Star Wars was filmed, George Lucas had been writing various scripts for a space feature in the stlye of the old Flash Gordon serials seen in the 1930's. Durring the inital concept of Star Wars he employed the artist Ralph McQuarrie to paint various scenes based on iideas from the not finished scripts. Some of these included areas of Tatoonie with the Jawas in places inhabitated.
These concept images were shown to the production crew as the base for the real desgins to be seen on film .
At the movies... where did we see them ?
Star Wars: Episode IV - A New Hope (released in 1977)
The very first appearance of the Jawas was in Star Wars IV 'A New Hope'. In their first scene, two Jawas encounter R2D2 in a rocky area of the Tatooine desert and caputre him using a ion gun, a few seconds later a larger group appear and carry R2 away. After fitting R2 with a restrinaing bolt he's taken on board their vehicle, where he meets back up with C-3PO. Subsequently they're taken to Luke Skywalker's Uncle Owen, as part of a droid sale.
Later on, the same group of Jawas are found murdered in the desert by Stormtroopers who were also looking for the 2 droids.
Some other Jawas are also seen outside the Mos Eisley Cantina and one is told by Luke to take his hands off his Landspeeder.
Star Wars: Episode VI - Return of the Jedi (released in 1983)
Another noteable appearance is in 'Return of the Jedi' where a Jawa in found as a slave in Jabba's the Paliace using a long fan to keep Jabba cool.
'The same Jawa i also seen later on Jabba's bardge
Star Wars: Episode I - The Phantom Menace (released in 1999)
There is a short scene where two Jawas are seen watching the Pod Racers go past them at high speed during the race.