The Korean language has a hierarchy of honorifics.
Different honorific forms are used for different ages, situations, intimacy-levels, and etc.
Amongst all, this Casual Version introduces the appropriate expressions used in the communications with your close friends, those very close to you, those with whom you have agreed on using the casual Korean, those in the same age as you, or those younger than you.
There also are some honorific forms for even more formal situations.