gdb considers registers to be a set with members numbered linearly from 0 upwards. The first part of that set corresponds to real physical registers, the second part to any pseudo-registers. Pseudo-registers have no independent physical existence, but are useful representations of information within the architecture. For example the OpenRISC 1000 architecture has up to 32 general purpose registers, which are typically represented as 32-bit (or 64-bit) integers. However the GPRs are also used as operands to the floating point operations, and it could be convenient to define a set of pseudo-registers, to show the GPRs represented as floating point values.
For any architecture, the implementer will decide on a mapping from hardware to gdb register numbers. The registers corresponding to real hardware are referred to as raw registers, the remaining registers are pseudo-registers. The total register set (raw and pseudo) is called the cooked register set.