For non-dormant seeds, germination starts when a seed is provided with water as long as the temperature is appropriate. The uptake of water by dry seed is called imbibition (imbibition means to drink: seeds imbibe water, you do not imbibe seeds). As seeds imbibe water, they expand and enzymes and food supplies become hydrated. Hydrated enzymes become active and the seed increase its metabolic activities to produce energy for the growth process. In addition, the water causes turgor pressure to increase in the cells and they are able to enlarge.
As you will see in the movies of germinating seeds, the first part of the seedling to emerge from the seed coat is the root (also called the radical). The emergence of the root is typically used as the first indication that a seed is viable. Eventually the shoot will also expand and emerge from the seed.
If germination occurs in darkness, root growth slows after the shoot emerges and shoot elongation accelerates. This behavior increases the chance that the seedling will emerge from soil into the light where it will be able to obtain energy from sunlight by photosynthesis. Once a seedling emerges into the light, the plant undergoes dramatic changes such as turning green and producing leaves. This light-dependent developmental transformation is called photomorphogenesis.