The terrans typically require a large variety of units to achieve victory. Check out terran unit combos.
SCVs possess the ability to repair all terran structures and any mechanical unit. This can be performed during or after battle, and your SCVs can be set to automatically repair any nearby damaged unit or structure. This ability allows terran players to hold aggressive pushes by repairing bunkers or to increase the efficiency of expensive mechanical units such as the Thor.
Medivacs can heal any damage sustained by biological units so long as they have energy available.
You can construct missile turrets to defend against harassment from enemy air units, support offensive pushes with additional anti-air firepower, and detect cloaked units.
Marines are one of the most versatile units in the game, and can be effective against all three races. Basic strategies generally will include a large number of marines. The stim pack upgrade increases movement and attack speed greatly at the cost of 10 HP, and combat shields provides an additional 10 HP. With these upgrades in addition to those from the engineering bay, marines scale well throughout the entire game and their effectiveness is further increased by massing them. However, they are very fragile, so enemy units with area-of-effect damage such as infestors, colossi, and siege tanks are able to take out large groups of marines at the same time very quickly. Be sure to build multiple barracks to keep marine production high, and do not forget a tech lab or two to research the important upgrades.
Medivacs are the marine's best friend, as they dramatically increase their survivability and efficiency. As soon as you survive past the rushing window, you may want to start building a factory and a starport. Add a reactor if you'd like to quickly stock up on medivacs. They also serve as transportation for up to 8 small units, 4 medium units, 2 large units or 1 massive unit (the Thor).
Terran units are upgraded based on where they are produced. Barracks units are upgraded at the engineering bay, while factory and starport units are upgraded at the armory. In addition, a tech lab constructed on each facility allows for tech upgrades such as siege mode for siege tanks or stim packs for marines. Because there are three different upgrade chains, one for each facility, it is extremely costly to upgrade the entire terran army. Therefore, strategies rarely upgrade all units at once and instead focus on maxing out one type before moving on.