HPS means high-pressure sodium. The bulb produces light, the ballast controls its electrical supply and the reflector distributes light over your plants. A complete HPS fixture brings these parts together. Check exactly which components are included before ordering.
For a new installation, start with the growing area, available height and heat removal. For a replacement, start with the exact fixture and ballast model. Matching wattage alone is insufficient: lamp technology, base and electrical version must also agree.
A replacement bulb and a new complete fixture solve different needs. Record which components you intend to keep, then assess the lighting as a complete system.