Using the Arduino Development Environment I can upload a sketch, or code to run on the Uno. According to the Reference guide I am using the Arduino boot-loader, this allows me to upload code without using any additional hardware. The boot-loader is active for a few seconds when the board resets; then it starts whichever sketch was most recently uploaded to the micro-controller.
This means of course I cannot save the last code uploaded to the Uno without more knowledge.
Let's say I have developed some code to interact with my environment, I upload the sketch say "alarm controller" and it runs looking at it's inputs until a breach occurs then does something, say sounds an alarm.
Fine, each time I want to run it I have to upload it via the Arduino Development Environment?
Where do I look for further instructions? Knowing the right questions is half the answer.