I've developed a simple website using TwentyTwelve and did not create a child theme (didn't know about that until recently). My confusion/fear comes from not clearly understanding what constitutes changes which will get overwritten when TwentyTwelve is updated. Since I'm such a novice, I haven't done any code work to the template that I'm consciously aware of. All I've done is installed several plugins (such as Styles, Black Studio TinyMCE, All In One SEO, Contact Form 7) and made other changes that only involve template options (such as choosing background colors, text colors, and so on). Is my website safe from the ravages of template updates or not? If you are wondering whether I've delved into the Editor area and changed any of that scary code ;-), the answer is no.
Thanks in advance to anyone who answers and is patient this beginner!