Terrans have many options when it comes to unit combinations.

Due to the importance the unit type/damage system has in StarCraft, you should carefully consider which units to field against the enemy based on their bonus damage types and abilities.

Terrans have seven units whose attacks deal bonus damage to particular unit types, which is more than the other two races (3 for zerg, 5 for protoss). Four terran units deal extra damage to light units: hellions, reapers, ghosts, and the Thor (anti-air attack). Three units deal extra damage to armored units: marauders, siege tanks, and vikings (anti-air attack). They are the only race in the game with two units whose primary purpose is harassment.

Reapers and hellions are fast, maneuverable units that deal bonus damage against light units like workers (prime target), zealots and zerglings. They are perfectly suited for hit and run operations, and can kill large amounts of light units rather quickly. Hellions are fast and can deal damage to many units at once (in a straight line). The reaper can bypass cliffs and deal good damage to structures (which is great for taking out important enemy infrastructure after dealing with the workers). As usual, battle conditions (blocked enemy ramp, bases accessible by land, enemy melee units with speed upgrades, your own resource counts, the main focus of your army, etc.) will dictate if you use one, the other, or a combination of both.

Marauders and siege tanks, on the other hand, are both armored units that deal bonus damage against other armored units. They can only hit ground targets, and both are based around high damage output. Marauders are mobile and can act as shock troops, while siege tanks are mostly used as heavy artillery to support terran infantry. The key differences between these units are attack range and damage, ease of massability (it is quite costly to mass siege tanks, and not always the best of ideas), and mobility. Counters to these units come in the form of large numbers of light melee units with speed upgrades, attacks from different sides, and catching the siege tanks while they are moving in tank mode.

Variety is important. A large group of marines, versatile as they are, will fare better if they are accompanied by marauders and tanks (or hellions if they are facing very large numbers of zerglings or zealots, for example). Marauders can dish out the pain against other armored units (like stalkers and roaches), but can only make a hasty retreat when confronted with void rays or mutalisks. A combination of marines and marauders is very strong against a straight-up ground army, but when facing tier three units such as high templar, colossi, or brood lords, it is necessary to mix in counters. Always be mindful of what units your opponent is using, and ensure that your army isn’t stuck in a disadvantage. Go forth then, adapt to the battlefield, mix it up, and celebrate your victory at the best spot in all of the nine worlds: Joeyray's Bar.

Sample Unit Combos

Marines, marauders, and medivacs (MMM). This is the most common combo.	Siege tanks, hellions and thor. Bringing in some SCVs to repair would complement this well. This combo is weak to armored air units.
Marines, medivacs, marauders, ravens, and a battlecruiser. Guess someone called in the fleet.	Thors with SCV, marauder and medivac support. A group of marines would increase this combination's anti-air capabilities.
Hellions, marines, medivacs, siege tanks. This combo is weak against heavy air units, like broodlords.	Marines, marauders, medivacs, siege tank, thor, and raven.
Thors, siege tanks, and hellions.	Marines, marauders, banshee, raven, medivacs, and hellions.