C++ is a superset of C that adds support for class-based object oriented programming and some other niceties, like templates, boolean variables, and it provides a real string class.
C compilers typically create simpler, faster, and lower level code that is much more suitable for things that need to be fast (e,g, OS).
Some well-known things created in C:
Linux
XFree86
GNOME/GTK+
Some well-known things created in C++:
QT/KDE
Fresco
other stuff cant think of now