Units are produced from structures. These units constitute your army and perform essential roles such as attacking and base-building. Most units consume supply, which limits the number of units deployed at once.
Many units have special abilities in addition to or instead of weapons. Units with multiple abilities that are used mostly for those abilities are called spellcasters.
Abilities are special actions of a unit that differentiate it from other units. They may be manually activated, conditionally activated, auto-cast, passive, or permanently active effects. "Autocast" abilities can be toggled to activate automatically when certain conditions are met.
Spells are manually-activated abilities that use up a certain amount of stored Energy. Spellcasters begin with 50 (although some units can be upgraded to start at 75) and generate up to 200. Generally speaking, spells in StarCraft II have extreme effects on units or the map and have no cooldown time, although they may have a cast time before they are activated, which sometimes leave the caster helpless. Some abilities and spells are present from the beginning of a game, and some must be unlocked with upgrades.
Peon is another term for Worker, inherited from WarCraft terminology. They are produced from the main base (or Townhall) for each race and cannot be made from any other buildings. They harvest resources and construct buildings. They have a weak melee attack that can not be upgraded but can still help in critical early-game situations (or all-in rushes).
Upgrades improve the functionality of units by improving their basic statistics or granting them additional abilities. Most upgrades are retroactive, i.e. pre-existing units benefit. Upgrades that affect a unit's starting conditions (such as the Infestor's Pathogen Glands upgrade) do not affect extant units. Upgrades take time and resources to complete. They are researched at associated technology buildings. Most upgrades require Vespene Gas as well as Minerals.
Weapon / Armor Upgrades increase units' attack damage or armor. Attack upgrades apply to each packet, i.e., units with multiple attacks will especially benefit from attack upgrades. Conversely, armor is applied to each packet, so armor is doubly effective against these units. For example, a Zealot attacking a Zergling with level 1 armor will deal 2 less damage, because his Psi Blades deal their damage in a packet of two attacks. Some units gain more than one damage per weapons upgrade, but armor upgrades always have a value of one. The upgrade factor is given in parentheses after the basic damage, e.g. 15 (+3). Each race has a pair of upgrade buildings dedicated to these sets of upgrades. They are the Engineering Bay and Armory, the Evolution Chamber and Spire and the Forge and Cybernetics Core.
These general guidelines do not necessarily apply to unit-specific upgrades; for example, the Ultralisk's Chitinous Plating upgrade adds two armor, rather than the usual one.
Some upgrades apply to individual buildings or units and change their functionality (referred to in-game as mutating, upgrading, morphing, or building units or buildings). They may require spending resources. Examples include the Command Center to Orbital Command upgrade,
Corruptor to Broodlord upgrade, and the Overlord's Ventral Sacs upgrade. They should be distinguished from transformations like turning a Gateway into a Warpgate, turning a Hellion into a Hellbat, or the Viking's different Modes.