#include is used to include outside libraries in your sketch. This gives the programmer access to a large group of standard C libraries (groups of pre-made functions), and also libraries written especially for Arduino.
The main reference page for AVR C libraries (AVR is a reference to the Atmel chips on which the Arduino is based) is here.
Note that #include, similar to #define, has no semicolon terminator, and the compiler will yield cryptic error messages if you add one.
This example includes the Servo library so that its functions may be used to control a Servo motor.
#include <Servo.h>
Servo myservo; // create servo object to control a servo
void setup() {
myservo.attach(9); // attaches the servo on pin 9 to the servo object
}
void loop() {
for (int pos = 0; pos <= 180; pos += 1) { // goes from 0 degrees to 180 degrees
// in steps of 1 degree
myservo.write(pos); // tell servo to go to position in variable 'pos'
delay(15); // waits 15ms for the servo to reach the position
}
for (int pos = 180; pos >= 0; pos -= 1) { // goes from 180 degrees to 0 degrees
myservo.write(pos); // tell servo to go to position in variable 'pos'
delay(15); // waits 15ms for the servo to reach the position
}
}