Quais são as diferenças entre Orcs e Uruks?

Uruk-Hais are Orcs. Just a special breed, that is stronger, faster and can withstand sunlight. Although Treebeard speculates that Saruman created them by crossbreeding Orcs with men, there is no tangible evidence for that. Nonetheless your average Orc is bow-legged while Uruk-hais have straight legs. Also they are bigger. But as far as we know Uruks are just a subspecies of Orcs.

This is further evidenced by their name: In the Black Speech Uruk-hai means Orc-Folk.

This question is a little bit tricky because it's important to remember that Uruk and Uruk-Hai refer to different things, at least in the dialect of the Isenguard Uruk-Hai. Tolkien was a linguist, and he loved adding this kind of cultural quirk. According to legend, he created Middle Earth, just as a place to simulate these kinds of cultural drifts, changes and misunderstandings.

Isenguard Uruk-Hai are strongly implied (in the book trilogy) to be an abominable hybrid of the races of Orcs and Men, designed by Saruman to have all the strengths of both and none of the weaknesses.

Meanwhile Orc, Uruk, Hobgoblin, Goblin and Snaga (lit. "slave" or "maggot") are all words that refer to members of the Orc race, which was created by the First Dark Lord, Morgoth, in the Pit of Angband.

One character might use one word in particular to refer to any Orc, because of that character's cultural background. Another character might use each of those words selectively, depending on the size, heritage and social status of the orc in question.

Some purebred Orc tribes even go so far as to claim the title of Uruk-Hai for themselves, adding another term for pure Orcs, and further complicating the task of distinguishing between Orcs and Hybrids. In universe, it appears that Saruman actually appropriated the term from them, when indoctrinating his hybrid shock troops, but it still makes things extra confusing.

From what I understand, Uruks are simply stronger and faster and can tolerate the sun. I also don't recall it ever being explicitly stated whether this is due to them being a different breed of, or just "better" orcs meaning rank wise, and their stature may be due to what their rank or position in orc society affords. The Uruk-hai on the other hand are definitely a product of Saruman as they are distinct from the other orcs in the raiding party which may or may not have also included Uruks from Mordor and Moria.

Uruk Hai: Strong, fast, tolerate sun, smart, disciplined, humanly proportional arms, poor eyesight at night

Regular Orc: Weak, slow, can't tolerate sun, idiot, undisciplined, longer arms, excellent eyesight at night

I think in the third book, when Sam and Frodo were under disguise, an Uruk was complaining about other orcs and stated all the things I listed here.

Although Saruman claims he perfected orcs, Sauron uses them as well.

Sauron created the black Uruk's, a race of black Orcs that are stronger. This is stated in the appendices of O Retorno do Rei when they attack Gondor. Saruman's Uruk-Hai are different, slightly. They have straight legs and are likely a cross breed of Men and Orcs. They are very similar to Sauron's Uruks, but different.

They both have Uruk's! Saruman's are called the Uruk-Hai. Sauron's are called the Uruks. They both have black skin.