Many seeds have ways to protect them selves  from  germinating at the wrong time of the year.  They have developed what is called dormancy. When the seeds environment is not suitable for germination, namely lacking moisture, oxygen, or it's to cold; the seed is in a quiescent state. Many seeds dispersed in the fall are not dormant but are quiescent because they lack water or the right temperature for growth, if they were provided water and a suitable temperature they would sprout. Quiescent seeds are not dormant.

Dormancy means that the seeds are alive but they need something more than just water and optimal temperature before they will start to grow. 
Most vegetable and annual flower seeds have been selected over many generations to eliminate dormancy and all they need is water and consistent temperatures in the 60-80F degree range and they will germinate.  But many perennial species seeds have dormancy conditions that need to be met before they will germinate.

Dormancy is caused generally by three different mechanisms:

 1) Chemical inhibitors that prevent growth.
2) Physical barriers that prevent the uptake or the movement of: water,  gases or chemicals with in the seed.
3) the embryo of the seed is not full developed and needs time after dispersal to ripen.

Some seeds can have both chemical inhibitors and physical barriers plus have undeveloped embryos or more than one type of each.
First we will deal with chemical inhibitors that prevent seeds from germination.

Plants that have developed dormancy in their seeds have done so for a number of reasons including challenges do to the environment they live in.  Plants that come from areas that have long cold winters  for the most part have some type of dormancy - once these seeds ripen in summer or fall there is not enough time for the seeds to germinate and the seedlings to grow large enough to over winter so dormancy prevents the seeds from germinating until spring.  The means to accomplish this are chemicals with in the seed that prevent germination,  these chemicals are destroyed by cold,  after the cold has dispelled the chemicals,  the seed is able to germinate and grow. Those plants that grow in dry regions of the world or were there are dry periods seasonally, their seeds  many times have dormancy requirements. Once the seed has come into contact with enough water to wash away the chemical compounds that retard germination they can begin to grow.  

The process of coarse is  much more complicated than the above lines would indicate- there are both chemicals with in the seeds that prevent germination and those with in the seed that promote germination and they work in tandem with each other.  Dormancy can be both primary in which the seeds do not germinate right away after ripping (the seeds need to dry to a certain moister content)  and secondary dormancy -  in which as time passes seeds develop dormancy of which "deep dormancy" or " after dormancy" is one factor.  An example of dormancy developing over time would be the Delphiniums or Monkshoods (Aconitum), fresh seed will germinate in one to three weeks while seed that has been dried and stored needs a cold moist period to beak down dormancy induced by chemicals with in the seeds.
The actual process of seed germination will be covered by a different page, at hand we are concerned with how to germinate our seeds and raise our seedlings into nice garden plants.