Let's see what we are going to do in this project in more detail:
First, we are going to build the hardware part of the project with a typical USB camera, a PIR motion sensor, and one SD card.
Then, we will write some code to test all the hardware connections of the project. We'll check whether the motion sensor is working correctly and try to take a picture with the camera while it is connected to the Arduino Yún board.
After testing the hardware, we are going to build the first application, which captures pictures whenever some motion is detected and automatically stores these pictures on the SD card.
Right after building this simple local application, we are going to connect the project to the cloud. The project will do the same as in the earlier case, take pictures when some motion is detected, but this time the pictures will also be uploaded to your Dropbox folder. This way, the pictures can be seen in real time from anywhere, as you can log in to Dropbox from any web browser...