Taking a first step, smiling for the first time, and waving "bye-bye" are called developmental milestones. Children reach milestones in how they play, learn, speak, behave, and move (crawling, walking, etc.).
Norfolk Ready By 5 has two summary early childhood development milestone cards (ages 0 to 2 (PDF) and ages 2 to 5 (PDF)) and there are several more extensive milestone resources:
Concerned about your child’s development? Don’t wait.