The Terran race is known as being very immobile because of their Siege Tanks and Thors which are notoriously slow.
This is all changing as players learn more about the Terran race and the type of advantages they can get with using Medivac drops.
Medivacs and infantry are extremely powerful if they are used in the right way at the right times.
The wrong way is to march a Medivac or two into your opponents base and drop your Marines off to do some damage.
Sure, this can work in some situations, but it's extremely unreliable.
A good player will pull his army over very quickly to kill your small Marine force and you will have effectively wasted 1000 minerals in just a few seconds (Assuming 2 Medivacs full of Marines) The right way to use Medivacs and Marines/Marauders is to counter attack and/or hit multiple places at the same time.
On many maps you can set up a nice defense out in front of your bases with Siege Tanks and Infantry.
This front force will protect against any frontal attacks that your opponent may throw at you.
Then you load up a few Medivacs with infantry and keep them ready.
When your opponent comes out to attack you, move in with your Medivacs to drop your troops into his main base and take out workers and key structures to cripple him economically.
If you're able to hold off the frontal attack, your drop will have put you far in the lead.
Even if your main defense falls you should be able to do enough damage to make up for it.
Another tactic is to load up 2 groups of Medivacs and drop in 2 different locations at the same time.
This works better late game as your opponent has more expansions and a bigger spread across the map.
Usually their newest mining base and main base will be furthest apart.
So if you drop one group at his new base and one group at his main you can do some serious damage.
If he chooses to defend his main he'll lose his newest base.
If he instead wants to keep his new base so he can keep his income up, then he'll risk losing his tech structures to your small infantry force.
Stimmed Marines and Marauders take down buildings extremely quickly.
When done right, Medivac drops can pull you back into the game if you're losing or help you get farther and farther ahead.
Use them wisely and you'll have another tactic to help you win as Terran.