The creation of the Uruk-hai is one of the more notable scenes from the Lord of the Rings trilogy, mostly for its revolting nature. Watching this giant form emerge from a pit of bubbling goo and slime and take the shape of a monster right before your eyes is something that burns itself into a person's brain. The Uruk-hai in general are such an interesting part of the series because they present a much bigger threat than the Orcs do.
But what is it that separates Uruk-hai from Orcs? Why were they created when it seems like Sauron's forces already had an abundance of soldiers with the legions of Orcs? And how were the Uruk-hai created in the first place? Tolkien's work has a few suggestions towards the answers but is often a little murky on the details. The film adaptations of The Lord of the Rings took a few liberties in order to make it much clearer how the two races differed, in order to make a much stronger distinction.
First of all, there are the obvious physical differences. Uruk-hai are much larger and beefier than Orcs, who are often more goblin-like. The Orcs were the soldiers of Sauron and the dark forces for a long time, but were one of the weaker forces of evil. Tolkien actually derived the word "Orc" from an Old English word that translates to goblin, which points to the fact that Orcs and Goblins are essentially the same things in Middle-Earth. There are actually a few sub-species of Orcs, including the Uruk-hai, who were stronger and were specifically created to be much better warriors than the Orcs that came before them.
Orcs were originally created in the First Age of Middle-Earth when the Elves were awakening. Melkor (also known as Morgoth) - who was the first Dark Lord and original source of evil in the world - captured Elves and turned them into Orcs through horrible torture. He did this as a way to insult the Elves because he was at war with them, at the time. Melkor was eventually defeated in this war, but his lieutenant Sauron survived and took over command of the Orcs. From there, they multiplied and became the original soldiers for the dark forces, and the most well-known to the general public.
Uruk-hai, however, were bred by Saruman in the Third Age in order to create a stronger army for Sauron in the War of the Ring. There are hints that Uruk-hai are the result of crossing men and orcs, to create a new breed that did not have the weaknesses of the Orcs. The Uruk-hai are stronger, larger, and can take the sunlight much better than the Orcs. In the Peter Jackson films, the Orcs actually conspire with Saruman at Isengard to create the Uruk-hai, as there are scenes of them overseeing the physical creation of the new soldiers.
Jackson made the clear choice in his adaptation to make the Orcs and Uruk-hai very distinct from one another in both creation and physical appearance, which not only makes it easier for the viewer to keep track of, but also lends a lot more diversity to the visual art style. It's much more interesting and dynamic to see the different species of Orc and be able to distinguish them visually, and it really makes the world feel more fleshed-out when there are so many different types of Orcs and Orc-variants.
The creation of the Uruk-hai also signals a movement towards the future, because they pose a much more significant threat than the Orcs. At the beginning of the story, it's scary for creatures like Hobbits to have to contend with Orcs when they've never been outside of the confines of the Shire, and it allows the audience to get a sense of the dangers of the world on a small scale. Then, when the stakes get higher, the enemies get stronger and scarier, which is where the Uruk-hai come in. There needs to be a bigger threat for the heroes so that the audience can see how they've grown and improved and can take on larger enemies than they thought possible.
It's interesting to see how Sauron's forces improve themselves through the creation of the Uruk-hai. They don't stay stagnant when they recognize that Orcs are common and readily available but flawed in their design, and Sauron works to find ways to extend his reach. Uruk-hai were created as a clear improvement on the previous army, as well as a way for Sauron to show his power and threat anew. The forces of evil are constantly innovating and finding new ways to try and thwart good, which is a really exciting dynamic in a fantasy adventure story. The villain always needs to keep improving and getting scarier, or else there's nothing for the heroes to learn or become stronger through. The step-up from Orcs to Uruk-hai was Tolkien's way of ramping up the stakes and the action for the heroes and showing the audience just how far they've come.