What is the difference between nucleation and crystal growth in crystallization?

Nucleation is the formation of new crystal nuclei (embryos) from supersaturated solution - it requires overcoming an energy barrier and can be primary (spontaneous or induced by foreign particles) or secondary (caused by existing crystals). Crystal growth is the enlargement of existing nuclei by solute addition to crystal faces. Operating conditions favoring nucleation (high supersaturation) produce many small crystals; growth-dominant conditions (lower supersaturation) produce larger crystals.