Physical development refers to a child's control over her muscles, her coordination, her ability to sit, stand, and eventually to walk and run. It also refers to her manipulative abilities. As she masters new skills, she will reject primitive ones. Physical development can be broken up in to three month intervals and divided into two categories: gross motor skills and fine motor skills.
Gross motor skills refer to the child's ability to control different parts of her body. Fine motor skills refer to her level of coordination of the different body parts (such as picking up a raisin with her thumb and forefinger). Milestones are punctuations in a baby's growth and development. Milestones of physical development proceed from head to toe.
Hence, the child first learns to control his head, then his body, arms and legs. These milestones provide a guide to parents telling them what to expect and when to expect it. Remember that departures from the norm should not be a reason for undue panic unless confirmed by expert medical opinion.
From what you have read in books and observed through others' experiences, you realize that a child's development should follow an approximate schedule. A note of caution. Every baby is unique and will probably not fit in to any theoretical parameters.