In that model of vehicle the side and curtain airbags were optional.
Side airbags deploy from the outboard sides of the seat - these deploy to protect your torso in a crash. Typcially these are found in the front seating positions only, however there are a few car companies that have them in the rear outboard seating positions as well.
Side curtain airbags deploy from the roofline to protect your head in a crash - these cover the 1st, 2nd, and often the 3rd row (if in a van etc).
In the 2003 Altima side and curtain airbags are optional. You can typically determine if a vehicle has side (torso) airbags by looking on the outboard side of the seat - there is usually a little tag that is labeled "airbag" or "side airbag". Curtain (head) airbags can be a little tricker - if it is equipped with them you might find a stamp that says "curtain airbag" on the inside of the vehicle by the roofline. You could also contact a Nissan dealer, they should be able to determine if your vehicle has the optional airbags by your VIN.