Command & Conquer/GDI units

Minigunner
The basic GDI infantry unit, cheap and quick to train but weak by themselves. They are best used en masse to overwhelm other enemy troops. They are most effective against similar enemy infantry and light vehicles, but should not go up against heavy vehicles without some strong backup, nor should they take on enemy infantry such as flame troopers.

Grenadier
Slower and slightly more expensive to produce than the minigunner, but they pack more of a punch. Unlike the minigunner, grenadiers should not swarm the enemy in close bunches, as their grenades can do devastating damage to themselves and their GDI allies. Space them out when defending against infantry and use them carefully against enemy vehicles.

Rocket Trooper
A very powerful infantry unit, but they move slowly and aren't cheap. Excellent for use against enemy vehicles and buildings when they are in large numbers, but their slow movement means they can get crushed by tanks if they don't get out of the way.

Engineer
Almost completely defenceless, but an important unit in the game. Used to capture enemy buildings, just make sure they have a good escort to protect them on their way.