Total newbie to Arduino here. I can't seem to figure this little detail out. In the following lines:

int ledPin = 13; // select the pin for the LED
int sensorValue = 0; // variable to store the value coming from the sensor

I don't understand the difference between assigning the variable 'ledpin' the value of 13 or assigning the variable name 'ledpin' to pin 13. seems like they are used interchangeably. In the next line they assign the variable 'sensorValue' a value of zero.

Thanks for the help,

"int ledPin" creates an integer variable (16 bit signed) named "ledPin", you use that name to refer to that variable.
"= 13" means you initialize that variable with the value 13 (decimal), 13 is the pin number of the built in led.
Later you would turn the led on with:

digitalWrite(ledPin, HIGH);

Same thing with "int sensorValue = 0;"

Int ledpin = 13; your assigning pin 13 of the Arduino(which is the LED mounted board), In your set up code you would have pinmode(ledpin,OUTPUT)//Sets pin 13 to an output or you could just have
// initialize digital pin 13 as an output.
pinMode(13, OUTPUT);
in your set up, it's just different ways of setting the pins up

Now int sensorValue = 0;, This could be a variable and your setting it to the value of 0 on start up but I think you want to read a value coming on pin 0 but this should read
int sensorValue = A0; // variable to store the value coming from the sensor
Then you would use int incoming_value = analogRead(sensorValue);//reads the incoming value on pin A0

You may want to take a look at the examples like blink & Readanalogvoltage, this gives you an understanding how to set pins up as input/output