First, try to recall how Russian typically expresses possession:
- У нас есть свой дом = We have got a house of our own.
- У моей бабушки есть машина, а у папы — нет. = My grandmother has a car and my dad doesn't.
- У тебя вопросы? = Have you got questions?
- У Саши насморк. = Sasha's got a cold.
The same у + Genitive can introduce a sentence about something related to or possessed by a person or an object (always without "есть"):
- У меня не работает мышка. = My mouse isn't working.
- У дома были заколочены окна. = The house's windows were boarded up.
- У моей жены брат — хирург. = My wive's brother is a surgeon.
You can interpret it as "The mouse I have isn't working", but essentially it is understood as if you just used a possessive construction ("моя мышка", "окна дома", "брат моей жены"). To my ear, it is a tiny bit less formal, but still quite neutral in style — I am sure I would not use in a legal document and probably in a scientific article but would not hesitate if I need it anywhere else.
Since Russian has a flexible word order, you can emphasize different words using both word order an intonation. So several options are valid:
- У стула сломалась ножка. (the most neutral/boring/mater-of-fact-ly)
- Ножка у стула сломалась.
- У стула ножка сломалась.
This device has its niche because it may be useful from a narrative point of view. Rather than stating facts about something belonging to something, you inform your listener that the sentence is about what happened to the object in question, what is interesting about that object, what the situation is.
"У меня мама заболела" is a bit more personal (it means "My mother is ill"), since it focuses on "your situation" ("As for me, my mother is ill"). "Моя мама заболела" is just the most straightforward way: anyone could replace "my mother" in teh sentence, and it would keep the same overall tone.
Another example. Say, someone's dear pet died, and their friend noticed they are sad. Then "У меня собака умерла" is a far more natural thing to say than "Моя собака умерла": after all, you are conmmenting on your situation, not on what your dog did.
UPD: Also, look at the first part of Anixx's answer. "Ножка стула" can truly mean just any leg of a chair or chair leg, including legs severed from the chair. Or those freshly made and not yet attached to any chair. So, it may mean the leg that is intended for use with a chair but not currently associated with any specific chair (similarly to how bath rug may be different from rug in the bathroom).
"У стула ножка/Ножка у стула ...." explicitly means the leg that the chair currently has — or at least had (has) at the moments you are referring to. An example of such use: «У моего кресла ролик заедает. Вот, я принёс — можешь глянуть?» (My armchair's castor gets stuck. Here, I brought it — could you please take a look? → here you mean that the castor jams when it is still attached).