The most important things in the universe are the living things, without them your empire would not exist. Your colonists are what makes your empire a strong empire.

Growing:
Each empire has different growth rate and climate demands. Some races like the Robots have tricks for colonist growth. Growth Rates vary from about 15 (Borg) to 175 (Crystals).

For maximum growth rate you want 101 turns worth of food and at least 150 happiness. Some buildings can increase and lower your growth rate(and even stop it). There are also exotics you can puchase to give you a nice bost.

Happiness:
You need to keep your colonists happy for them to grow and pay taxes. To keep them happy make sure they have plenty of food (for max growth rate you want 100's if not 1,000's of spare food), keep the number of mines and smelters low(49, 4). Avoid building labor camps and labor mines on important populated planets. They can cause the happiness to drop and vandalize the base if they riot. Build base structures that increase happiness like Government centers, public space ports, and results. Last but not least keep other empires from lowering your happiness. Having 101 med units helpsa little too.

Some empires have special abilities that can raise or lower colonists happiness.

Each empire colonists accept different tax levels, and pay different amounts in taxes. Solar Federation colonists, for example, prefer enslavement tax levels. But most races don't like such high tax rates.

*Cyborg happiness is always reset to 100.