There are three races in the game: Sapiens (humans), Titans (machines), Khraleans (bugs and insects). The races differ in strategy, firepower, armor, and speed. For example, the Sapiens have the fastest mobility out of the three, while Titans have the toughest armor and Khraleans have the deadliest firepower in terms of strength in numbers. Each race is more advantageous than the other in this order:
Sapiens are better than Khraleans because they're too fast for the Khraleans to deal heavy firepower.
Khraleans are better than Titans because their firepower in numbers is too much for them to handle.
Titans are better than Sapiens because their armor is too tough to crack for conventional weaponry.
To help offset this advantage, each race is given a support unit each to be able to convert enemy (and allied as well in team games) infantry for their side. In this order, Engineers reprogram Mechas, Infectors infect Marines, and Assimilators assimilate Underlings. With the right strategy, each race can beat all the other races. It takes time to learn the right strategy but the best way to learn is through practice.