They all use the same GPUs. A Geforce 770, for example, uses the same exact 770 chip from Nvidia regardless of whether it's made by ASUS, EVGA, or whomever. The same for a given model of AMD Radeon card.
They can differentiate themselves with factory overclocks, the aesthetic look, and most importantly, the cooler used. The cooler on different manufacturer's cards is often custom designed by that manufacturer and can offer different performance and loudness.