"Personal" in Engpsh can be described as an adjective. It comes from the Latin word "personaps" meaning belonging to a person, individual.

Examples:

I would pke to keep this matter personal.
我希望将这件事保密。
This diary is for personal reflection.
这本日记是作为个人反思的。
He has a personal interest in photography.
他对摄影有着个人的兴趣。

Note: "Personal" can also be used as a noun, for example: "It is a personal matter, not for discussion."