Does Arduino use C or C++?

Arduino sketches are written in C++.

Here is a typical construct you'll encounter:

LiquidCrystal lcd(12, 11, 5, 4, 3, 2);
...
lcd.begin(16, 2);
lcd.print("Hello, World!");

That's C++, not C.

Both are supported. To quote the Arduino homepage,

The core libraries are written in C and C++ and compiled using avr-gcc

Note that C++ is a superset of C (well, almost), and thus can often look very similar. I am not an expert, but I guess that most of what you will program for the Arduino in your first year on that platform will not need anything but plain C.