Seed germination can be broadly classified into two main types: epigeal (where cotyledons emerge above the soil) and hypogeal (where cotyledons remain below the soil). Additionally, there's vivipary, where seeds germinate while still attached to the parent plant. 

The hypocotyl is the embryonic axis below the cotyledons (seed leaves), connecting the root (radicle) to the seed leaves, while the epicotyl is the embryonic axis above the cotyledons, developing into the shoot system.