Development is a gradual multi-step -highly regulated- process by which a complex multicellular organism arises from a single cell (the zygote). 

Major processes involved in animal development are:

• cell division -> increase in number of cells 
  
• cell migration -> change in spatial location of cells
• cell differentiation -> diversification into specific cell types
• pattern formation -> tissues and organs develop in the correct place and orientation within the embryo. Cells acquire different identities based on their spatial location. Cell identity is initially partly defined within broad territories of competence, and becomes progressively refined as tissues and organs are formed. 
• morphogenesis -> generation of the complex shapes and structures that characterize the adult body organization. 

For a general overview on the stages of animal development refer to the online version of  the book "Developmental biology, 6th edition - Chapter 2 - The Circle of Life: The Stages of Animal Development".