There are many seeds that will grow into a beautiful seedling in no time, only depending on the right humidity and temperature. Little attention is required for this, but that is certainly not the case with every seed variety. There are also some seeds depending on the natural conditions in which seeds can germinate and we have to make sure we imitate that if we want the seed to germinate.
There are light germinators, dark germinators, seeds that need stratification (cold germinators) and seeds that need scarification (damage to the seed shell).

Temperature.
The right temperature is one of the most important factors in seed germination.Temperature has a strong influence on germination, the germination percentage, but especially the germination speed of the seed.
The differences between the different types of seeds are sometimes very large.

Humidity.
Seed need to absorb moisture to initiate germination. But once germination has started, the seed must not be allowed to dry out because that is catastrophic for the germination process.
However, an excess of water can also have a negative effect.Soaking seeds before germination can promote moisture absorption, making the seeds germinate better and faster. This is especially true for the coarser seeds.

Light germinators.
The seeds need light to germinate. Same varieties really need light but other varieties just benefit from light to improve the germination.Light germination seeds are often small and should be sown on the soil surface. Gently press them into the soil, do not cover them, ensuring good seed-to-soil contact but without burying them too deeply. Make sure the seed remains moist.
Best method is to cover the seeds with a thin layer of fine vermiculite. Vermiculite is porous enough to let light shine through, while retaining enough water to stay in place and keep the seeds and soil under it moist.
Examples of light germinators: Lettuce, Petunia and many more.

Dark germinators.
There are also seeds that germinate in the dark. Same varieties really need darkness  but other varieties just benefit from darkness to improve the germination.Light actually inhibits the germination process. The seeds are covered by a layer of soil.
Examples of dark germinators: Cyclamen, Digitalis and many more.

Cold germinators.
Seeds need a cold treatment for activating the germination. Often the seeds need first a period of several weeks in a warmer condition and need a cold period after that. Best procedure is to have your seedtray covered with snow or place the seeds in humid sand in a bag in the fridge for several weeks. After that the seeds should germinate normally.

Air.
It is also important to have sufficient air around the seed. Provide a sowing soil that is sufficiently airy and not too compacted. Germination is strongly inhibited by soil that is pressed too hard or too wet.