Yes Infinity is the luxury brand for Nissan in the US. Same deal as Acura and Honda or Lexus and Toyota. Only the Honda's are actually using the same cars and trucks for both brands. Toyota/Lexus and Nissan/Infinity share the trucks but the cars are different (except for the FWD Lexus car)

The closest Infinity to a Altima is the G37. However they are completely different cars. The Altima is FWD and the G37 is RWD.