Traditional unit Tiers were based on the Warcraft 2 model where you upgraded your main worker producing structure to get new units. Since both races had 2 possible upgrades, this lead to 3 tiers.
Starcraft 1 mimic'd this with the Zerg race but not all races. However, the terminology had stuck for how to describe various phases of the game. Tiers referred to how deep into the tech tree a player had gone and how much resources/time they had spent on tech. As a result a general terminology emerged, placing most units in a tier.
With the advent of Starcraft 2 and the addition of a number of earlier units, there has been a rise in referring to certain units as tier 1.5. These are units after tier 1, but still before tier 2.
Regular terminology is detailed as follows:
Tier 1
Tier 1.5
- Roach, Baneling
- Marauder, Reaper
- Stalker, Sentry
Tier 2
- Hydralisk, Infestor, Mutalisk, Corruptor, Overseer
- Ghost, Hellion, Siege Tanks
- Immortal, Phoenix, Void Ray, Observer, Warp Prism
Tier 2.5 (sometimes referred to as tier 2 or 3)
- Banshee, Raven, Viking, Medivac
Tier 3
- Ultralisk, Broodlord
- Battlecruiser, Thor
- Colossus, Mothership, Carrier, High Templar, Dark Templar
You'll note that tier 2.5 has only terran units, and is often just referred to as tier 2. This is true for all of 2.5 except for the Raven. Because the Raven requires so many upgrades to be effective, its sometimes referred to as Tier 3 and others as Tier 2. Its also worth noting there is some discrepancy around which tier ghosts should belong to.
These naming conventions are not absolute, but are common amongst professional casters (people who comment on professional games).
Best Answer
To quote Artosis