at tænke noget om noget/nogen = to think something about something/somebody (i.e. to have an opinion about them)
For example: Hvad tænker(/synes) du om hans nye hat? = What do you think about his new hat?
The answer would most likely start with: "Jeg synes den er..."
at tænke på noget/nogen = to think about something/somebody
Jeg tænker på hende hver dag = I think about her every day
Han tænker ikke på andet end mad = He doesn't think about anything other than food