It ain't just a ring; that much we know. But what is it exactly?
During their adaptation of Tolkien's novels, Jackson and crew decided that the Ring couldn't just be an object; it also needed to be a character. So they gave it a creepy voice, and gave it the pretense of a physical weight (using the musical score and magnets in the floor of Bag End) to match the ethereal power it holds over those who possess it. They also connected it to the all-seeing eye, merging the two manifestations of Sauron's surviving essence.
So the Ring begins to take on a life of its own, subtly working it's will into the world as it slips of the finger of Isildur, leaves Gollum, slips onto Frodo's finger as he falls, and constantly calls out to those around it, seducing them to wear it so that it may return to its master.
But the Ring isn't only its own character, it's also a symbol of power and greed and deception.
In the grand scheme of things, the Ring is just a tiny object that has floated around for close to three millennia. But from this small piece of jewelry comes a death threat to Middle-earth. The Ring, as we see from the opening sequence with Sauron incarnate, is full of a physical energy, a force that can literally lay its opposition flat in a single blow.
But its power also lies in its ability to deceive. The Ring cannot simply roll into Mordor; it would have no hope in reaching its master if it could not trick the greedy minds of the unsuspecting—the minds of Saruman and Boromir and Bilbo and, at times, even Frodo. You may have heard that "absolute power corrupts absolutely" and the Ring is a testament to evil that power-hungry beings can unleash.