I see people referring to a StarCraft 2 unit's tier (1, 1.5, 2). What does this mean, and how can I tell what tier a unit is?
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 2.5 (sometimes referred to as tier 2 or 3)
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).
Essentially, a new "tier" is unlocked when a key building in the tech tree has been built. Units that can be built as soon as you have the first structure (such as marines, zealots, and zerglings) are tier 1. Stalkers are tier 1.5 because they also require a cybernetics core, and Void Rays are tier 2 because they require a Stargate.
As for how to tell what tier a unit is in, the only real way is to memorize the tech tree. The terminology is rather meaningless unless analyzing build orders and strategies; if you fluidly upgrade and build new buildings then you're not going to notice a big distinction.
EDIT: With Zerg it's easier to tell the tiers because you have to upgrade to a Lair and then a Hive which is much more concrete.
With Zerg, it's the most clear. You can tell by the level of the hatchery:
With other races, it's a little more ambiguous, and some people call things differently, but generally the division is something like this: