It is Russian equivalent to the French "chez".

1. у + genitive case for possession ◆ belonging, relationship to someone, something ◆ У меня в квартире газ, а у вас?
2. у + genitive case for measures and quantities ◆ a part of something ◆ У кошки черыре ноги.
3. у + genitive case for location ◆ being near to something, someone, at the house of ◆ Я была у подруги. У её дома остановилась машина. ◆ find out from someone ◆ У кого бы мне узнать, как это сделать?

Motion to-towards-onto-into	Location on-in-at	Motion from-off-out-of
– Куда? – B, НА + accusative	– Где? – B, НА + prepositional	– Откуда? – ИЗ, C + genitive
– К кому? К чему? – К + dative	– У + genitive	– От кого? От чего? – ОТ + genitive

• If B is used to express motion in prepositional and accusative then ИЗ is used in genitive
• If НА is used to express motion in prepositional and accusative then C is used in genitive