In simple terms, breast buds are the early breast tissue that first appears in adolescence.
In the medical community, the breast development process is referred to as “thelarche,” which is pronounced “thee-LAR-key.” (This term can come in handy if you ever want to research the issue on the Internet. If you type in “thelarche,” you’ll be directed to good, informative medical Web sites. However, if you type in any search term that includes the words “women’s breasts,” you’ll be directed to Web sites that will make you blush.)
The appearance of breast buds will typically occur around the same time when you undergo a "growth spurt". This budding process begins when a hormone known as estradiol (an estrogen hormone produced by the ovaries that is responsible for breast development and menstrual cycles in females.) starts coursing through your body. It’s this hormone that causes your once boy-like figure to begin hinting at its future femininity.
Growth Spurt is a period of rapid growth in height, weight, and muscle mass that takes place in adolescence, typically between the ages of 11–14 for girls, and 12–16 for boys