1: All nouns in German have a grammatical gender. There's mostly no reason that any word has a particular gender, that's just the way it is. There are some patterns, for example certain word-endings can indicate a certain gender, but generally there is no way to figure out if a word is masculine, feminine, or neuter without just learning each word by heart. Der/die/das/den/dem/des are all just words for "the" in different genders and cases. Der Stuhl is der because Stuhl is a grammatically masculine word. Die Milch is die because Milch is a grammatically feminine word. Das Brot is das because Brot is a gramatically neuter word.
#What is the difference?
stoic echo
2: Der Stuhl and den Stuhl are different in the same way that the words he and him are different. They mean the same thing, but one is used when the thing you're talking about is the subject of the sentence (He is going for a walk. He ate breakfast. He has a dog) and the other is used when the thing is the object in the sentence (I love him. I spoke to him about the homework. I gave him a present). Articles change both based on gender as well as what role the noun plays in the sentence. This is called case.
Der Stuhl means the Stuhl is the subject of the sentence. E.g. "Der Stuhl is aus Holz" - the chair is made of wood. Den Stuhl means the Stuhl is the object. E.g. "Ich sehe den Stuhl" - I see the chair