user14669: When I bought my Arduino Uno and connected it to my computer, I tried to make the simple blink circuit (before programming the Arduino) and suddenly the LED started blinking. Is the blink example the default Arduino sketch?  
I am sure that I didn't uploaded anything to the board.

user4143: Some may have the "Blink" sketch pre-installed, but many don't. However, when there is no sketch installed the board sits in bootloader mode waiting for you to upload a sketch.  While it's in bootloader mode the LED pin pin D13 blinks.
If your LED is blinking about once a second then it's the Blink sketch.  If it's rapidly blinking three or four times per second or so then it's in the bootloader and you have no sketch loaded.

14669: <p>When I bought my Arduino Uno and connected it to my computer, I tried to make the simple blink circuit (before programming the Arduino) and suddenly the LED started blinking. Is the blink example the default Arduino sketch?  </p>

<p>I am sure that I didn't uploaded anything to the board.</p>
 4143: <p>Some may have the "Blink" sketch pre-installed, but many don't. However, when there is no sketch installed the board sits in bootloader mode waiting for you to upload a sketch.  While it's in bootloader mode the LED pin pin D13 blinks.</p>

<p>If your LED is blinking about once a second then it's the Blink sketch.  If it's rapidly blinking three or four times per second or so then it's in the bootloader and you have no sketch loaded.</p>
