Having taken snippets and applied them for use in my code from the 'blink without delay' example, could somebody please explain to me the use of 'unsigned long' in place of 'int'? It seems 'int' takes up just a few bytes more, but I'm not sure what 'unsigned long' really is, or indeed why the example uses it. English please? 
const int tempPin = 1; // defines the LM35 input into analogue pin 1
int valFan = 0; // creates an integer used for temperature mapping, this being mapped fan speed - default to 0
const int fanPin = 9; // defines the fan output connected to digital pin 5
const int onboardLED = 13;
long previousMillis = 0;
const unsigned long interval = 500;