Daggoth: the Tiamat Brood produces zerglings, hydralisks, lurkers, ultralisks. Daggoth has 10 more supply than other cerebrates.
Araq: the Jormundgand brood produces mutas, guardians, scourges, overseers. All its units move 10% faster.
Nargil: the Fenris brood produces zerglings, hydralisks, defilers, overlords with ventral sacks. The Fenris brood isn't limited by a control zone, and the only ones which needs to built overlords for control. While in Defensive mode will defend other cerebrates.
Zasz: the Garm brood produces defilers, flying queens, hydralisks, overlords with ventral sacs, overseers. All friendly biological units close to Zasz regenerate energy faster. Unlocked at level 6.
Kagg: the Surtur brood produces drones, zerglings, hydralisks, lurkers, guardians, sunken colonies, spore colonies. Has a broader control area for Defense mode. Defensive structures cost half supply each. AI will leave defensive structures near other friendly buildings. Unlocked at level 6.
Gorn: the Baelrog brood produces guardians and devourers, (born as mutas but AI morphs them fast), ultralisks. Cerebrate has 1000 life and 3 armor. Mutas are weaker but are born extra fast. Unlocked at level 6.
When the Cerebrate upgrade cache is unlocked, the upgrades are the following: Daggoth - spend less resources to build army (100/100); Araq - units spawn scourgesplitters on death (even scourges!) (200/200); Nargil - builds Nydus Canals between Nargil, other cerebrates and the Overming (200/200); Zasz- increases energy regeneration bonus (100/100); Kagg - killed units spawn broodlings (100/100); Gorn - spawns a supply-free Torrasque which ressurects at Kagg if carcass is destroyed (300/300). Those upgrades only affect the Cerebrate's own units.