Your iPad has a couple of mechanisms designed both to save your battery life and to make the iPad a green and energy efficient device: the first, which you've surely noticed, is the dimming of the screen after a certain period of inactivity. Since the iPad display is what typically consumes the most amount of power on an ongoing basis, letting the screen dim when it does can save you quite a bit of battery time before your iPad has to shut down. The second power-saving feature is that your iPad automatically goes to sleep after a bit more time; but, as you'll learn in this tutorial, you can save even more battery life and power by manually putting your iPad to sleep yourself when you no longer need to actively use it (and you can of course also turn off your iPad altogether).
Manually make your iPad sleep
While some apps may contain a "software" version of the sleep function, the hardware button for sleeping will work all the time, regardless of the app you happen to be using at the time. The screenshot below shows you highlighted the "Sleep" button of your iPad: to put the device to sleep, just press once on that button, and the screen will go blank and dark (your visual confirmation that the iPad has now gone to sleep). And, unless you have disabled and turned off iPad sounds, you will hear a distinct click noise playback as soon as you click on the Sleep button.
Wake up your iPad from a sleep state
When your iPad is asleep and you want to wake it up, you now have to choices: you can either click on the same "Sleep" button outlined above in the screenshot, but you can also use the "Home" button, as shown on the screenshot on the left. When your iPad is in a sleep state, either button will achieve exactly the same effect, so which of these you use is entirely a matter of personal preference. In both cases, you will see the "slide to unlock" prompt.
iPad Tip: if you have accidentally woken up your iPad from a sleep state, no need to slide the "slide to unlock" button, you can right away click on the "Sleep" button, and your iPad will return to sleep mode.