From the Halo 3 Bestiary book which describes the various races, their homeworlds, and their level of technological development:
Tier 7: Preindustrial
Tier 7 is one of the most common and stable states, with limited weaponry and environmental threats. Societies tend to be small and scattered, driven by subsistence farming, foraging, or hunter-gathering needs. Technology is limited to simple tools, weapons, or agrarian implements and methods, but a basic understanding of planetary or solar mechanics is not uncommon.
The Flood are the only race in the galaxy with this tech level natively, though they have a Tier 2 adopted tech level. This would seem to suggest that they make nothing of their own, just hijack what they can from their hosts.
Tier 6: Industrial Age
Tier 6 is often the pinnacle for a civilization. Agrarian societies can remain stable in the preindustrial stage, but Tier 6 population strain and mechanized food production invariably create political and economic pressures few can balance. Moving past this usually promises advancement. Some societies improve environmental and medical understanding concurrently with mechanical and transport advancement; those that do not are frequently doomed.
The native tech level of the grunts before being introduced into the Covenant.
Tier 5: Atomic Age
Tier 5 species usually become space-farers, focusing on clean energy production. The occasional belligerent species will use atomic energy for weapons, often resulting in extinction. In-atmosphere craft are a hallmark, often leading to spaceflight.
None of the known Halo species are on this level.
Tier 4: Space Age
Tier 4 is often the final resting place for species intelligent enough to break free from their cradle's surface only to fill the gulf surrounding it with war. Their comfort-focused technology can include medical advances.
Note: Jiralhanae are the only species on record who achieved space, reduced themselves through internecine war to a preindustrial condition, clawed their way back to their former state, and learned nothing from the experience.
This is the tech level most of the covenant races achieved before they became members. Brutes (Jiralhanae), Drones (Yanme'e), and Jackals (Kig-Yar) are all Native Tier 4.
Tier 3: Space-Faring
Humans have efficient slipspace navigation, mass drivers, asynchronus linear-induction weapons, holocrystal storage, and semi-sentient AI, but their creation requires blood sacrifice, memory transfer, and flash cloning. They have had little or no outside influence- until recent events. Since Covenant contact, they have been on the verge of huge artificial advancement.
The native tech level of Humans and Hunters.
Tier 2: Interstellar
The Covenant's accurate slipspace navigation, near instantaneous interstellar communication, and man-portable application of energy manipulation owe everything to Forerunner technology.
Note: The two primary extant interstellar civilizations were space-faring prior to discovering Forerunner artifacts- or each other- but they have not successfully reverse-engineered those artifacts.
The Elites and Prophets are both at this level.
Tier 1: World Builders
Forerunner ability to manipulate gravitational forces, create AI with full sentience, fabricate super-dense materials, and perform ultra-accurate slipspace navigation is unequaled by any extant civilization.
Forerunner tech level.
Tier 0: Transsentient
As the Forerunners had no examples of civilizations with technological accomplishment greater than themselves- with the exception of the Precursors- this is a theoretical ceiling. They can travel intergalactically and accelerate evolution of intelligent life. These may be creatures of legend.
It also has an entry regarding the Covenant Engineers which I've seen get some flak here. According to the bestiary book (which seems to be written from the perspective of some manner of outside observer, possibly forerunner). They're forerunner constructs who's nanomechnical structures are indistinguishable from their biological analogues to those races without Tier 1 biomedical tech built as maintenance crews.