Tech is how unit strength is defined, it is the "levels" of Supreme Commander. it is common to see units referred to as "Tech 3 Bomber" instead of by name, such as "Sinntha". Knowing the difference in strengths and abilities of your current tech is of strategic importance.
Tech 1 is the basic starting level in a game of Supreme Commander. The ACU you start with is capable of building most tech 1 buildings, with a few exceptions. This includes all 3 factories, which produce tech 1 units. All 3 factories are capable of producing tech 1 engineers and a small selection of units.
Units common to tech 1 land factories are scouts, light assault bots, medium attack units (usually a tank although Cybrans get the Mantis bot instead), light artillery and light mobile AA units. The only exception is the Seraphim factory, which has scout and light assault bot combined in a single unit, the Selen.
Common to all 4 air factories are air scouts, interceptors, attack bombers and light transports; the Cybrans also get the Jester light gunship. Common to all naval factories are frigates and attack submarines. However, the aeon get a T1 attack boat to compensate for their frigate lacking AA. T1 structures are all virtually identical among the factions at this stage. T1 point defense is also basically the same, but the Aeon and Seraphim point defenses don't need to turn, and so they are slightly more resistant to kiting than their UEF and Cybran counterparts.
To advance in Tech levels, you need to either upgrade your factory, (which will then produce Tech 2 units, notably the Tech 2 engineers) or your ACU, though this is not recommended due to the cost.
Tech 2 is your second level of tech. This level is full of heavier defenses, numerous support buildings, and important combat units. The advent of T2 marks the part of the game where units begin to pose a serious threat to an ACU other than simply by overwhelming numbers and the factions strengths and weakness' begin to diverge more markedly.
Common T2 land units include the heavy tank, amphibious tank, mobile missile launcher and mobile AA flak unit, although the Seraphim get an assault bot (the Ilshavoh) instead of a heavy tank. Truly unique units start appearing at this stage such as the UEF Sparky combat engineer and the Cybran Fire Beetle suicide bomb.
All factions get a torpedo bomber, medium air transport and gunship and the UEF, Cybran and Seraphim get a fighter-bomber, while the Aeon instead get the Swift Wind combat fighter, but compensate with the Mercy guided missile.
Common naval units include the destroyer and cruiser, marking the start of effective naval-shore bombardment strategies, while the UEF, Aeon and Cybran get submarine hunters. The UEF and Cybran also get the unique Bulwark shield ship and CI:18 Mermaid counterintelligence boat respectively.
T2 structures bring a whole range of powerful attack and defence options, including light artillery installations, tactical missile launchers, shield generators and tactical missile defences, as well as upgrades to the T1 structures such as point defences, AA emplacements and radar. The UEF and Cybrans also gain access to their powerful engineering stations, the Kennel and the Hive at this level.
This is the maximum level your ACU, factory, Mass extractors, Shield Generators, and radar/sonar can be upgraded to. Tech 3 contains the heaviest units a factory can produce as well as the greatest diversity between factions; this is the level where ACUs can be destroyed in a single coordinated strike by the units.
The only common T3 land unit between the factions is their mobile artillery pieces. Seraphim get their T3 mobile shield, Aeon get their shield disruptor, and the Cybran and UEF get their armored assault bot. The human factions get their heavy assault bots while the seraphim get their siege tanks. T3 air factories produce spy planes, powerful air superiority fighters (ASFs), strategic bombers and heavy gunships (except for the Seraphim); the UEF also benefit from the Continental heavy air transport and the Aeon get the Solace heavy torpedo bomber. T3 naval units are extremely expensive and include strategic missile submarines and battleships (again, except for the Seraphim who get a T3 submarine hunter and a strategic missile battleship instead) and aircraft carriers (with the exception of the UEF who get the Atlantis experimental instead). The UEF also get the Neptune Class battlecruiser and the Aeon have access to the Torrent Class missile ship.
Finally, T3 engineers are capable of building Quantum Gateways capable of transporting powerful SCUs to the battlefield; these units cannot be built by normal factories. T3 structures feature powerful and expensive weapons such as heavy artillery installation and strategic missile launchers, as well as strategic missile defences, as well as improved shield generators for most factions (except the Cybrans). Some heavy defences are introduced such as the UEF Ravager point defence and the Cybran HARMS torpedo system, while all 4 factions get T3 AA emplacements.
Tech 4 is not a real tech level in the same sense as the other 3, it is an unofficial term for Experimental units that can only be built by units with a T3 engineering suite (T3 engineers, SCUs and upgraded ACUs). Large, devastatingly powerful units too big to be constructed in factories, these units are extrordinarily expensive but can single-handedly turn the tide of battle. Several of them are sufficiently powerful enough to be referred to as Game enders, capable of deciding an entire match on completion.