As Gamow recounts in his autobiography, the name was chosen in part to commemorate the gambling establishment where the two physicists had first met, and "partially because the Urca Process results in a rapid disappearance of thermal energy from the interior of a star, similar to the rapid disappearance of money from the pockets of the gamblers on the Casino de Urca.
The baryons can be nucleons (free or bound), hyperons like Λ, Σ and Ξ, or members of the Δ isobar.
The Urca process is especially important in the cooling of white dwarfs, where a lepton (usually an electron) is absorbed by the nucleus of an ion and then convectively carried away from the core of a star.
Because the neutrinos emitted during this process are unlikely to be reabsorbed, this is effectively a cooling mechanism for white dwarfs.
If a neutron star contains a central core in which the direct Urca-process is operative, the cooling timescale shortens by many orders of magnitude.