Receiving quite a bit of screen time in the films but not much explanation are the Palantíri, or Seeing-stones (Palantír being the singular form). These glassy, bowling ball-like orbs are an ancient means of visual communication in Middle-earth and cause much confusion in the War of the Ring for all parties involved.
The Seeing-stones were created in the Elder Days by the Elves, perhaps Fëanor, the creator of the Silmarils, and gifted to Men while Númenor still existed. After Númenor’s destruction, Elendil preserved the Stones by taking them to Middle-earth. They were scattered abroad across the kingdoms of Gondor and Arnor in important places such as Weathertop, Minas Tirith, Minas Morgul, Osgiliath, among others. There were seven remaining after the Fall of Númenor and more were lost as the Third Age went on. Some were lost when the northern kingdoms fell, also when Minas Morgul was taken and during the Kin-strife at Osgiliath.
By the time of the War of the Ring, only the Palantír at Orthanc in Isengard, the secret Stone at Minas Tirith, and the Stone taken by Sauron from Minas Morgul remained relevant. These interesting devices could perform specific operations that aren’t fully explained in The Lord of the Rings, but the information can be found in the extended works. There still remained a Master-stone in the Undying Lands that could be used to see all the others, but those in Middle-earth had more limited abilities. No sound was transferred between them but pictures were. Only the strong-willed could use the Stones with efficiency and they were not immune to deceit.
For instance, Sauron fooled Saruman into thinking he could share power with him if the Wizard joined him by communicating with him through the Palantír in Orthanc. Sauron also deceived Denethor in Minas Tirith so that he could see no hope of winning in combat against the might of Mordor. Sauron was also deceived, however, when Pippin looked into the Orthanc stone after the Company obtained it. Not revealing much else other than he was a hobbit, Pippin accidentally made Sauron think he was the Ring-bearer, which proved fortunate for Gondor and Frodo as Sauron would not be expecting the Ring to already be within the borders of Mordor, where the Ring actually was at that time.
There was a particular way to look into the Stones and only those with sufficient mental strength could see images clearly either way. If you wanted to view something in the East, you would stand on the West side of the Palantír and look through it into the East. Communication between each Stone could be achieved by orienting them towards one another and the users could perceive each other’s thoughts in their heads, though no other sound could be transmitted. “Zooming in” on particular locations and objects could be achieved through heavy concentration but this was mentally and physically taxing and not typically done unless the circumstances demanded it.
The Stones varied in size and “distance” they could see. The larger ones were typically more powerful but there were exceptions and the strong-willed could use the Stones as they wished for the most part. The smallest were about one foot in diameter and the largest required multiple men to carry.