It sounds so good!
It's been called a lot of different things. The Beatles called it a twin track, while others call it multi-tracking and still others call it layering or stacking. When used with vocals, the effect has been described as a thickened or widened vocal. When used with other instruments, it provides a chorus-like, stereo-spread or a widening effect. Let's jump right in.
There are so many tools at our disposal when it comes to creating music however, double tracking is one of the best ways to add some real power to vocal tracks. There are several strategies to double-tracking vocals, my primary one has been to duplicate the vocal track twice creating left and right tracks sitting beside the original vocal (center) track. I then add digital delay to the left track and reverb to the right track, while maintaining the center track clean. Next, I pan the left track all the way to the left and the right track all the way to the right. I then balance the levels of all three tracks, keeping the center vocal out front and the delay and reverb tracks slightly behind.
So, what I have effectively done? I have thickened the vocal by creating more voices and using different effects in a stereo application. The reverb and digital delay effects bounce back at different rates, creating more space. Our brains interpret the vocals as being from multiple singers, as each voice is slightly different. Hard panning the vocals left and right simply reinforces that fact.
BMI.com published an article called Double Tracking Explained, which provides some additional insight into double tracking. Here are some thoughts I picked up from the article. First, the method I use duplicates the vocal track, you can also record the doubling tracks live. Some mixing engineers will have the vocalist cut multiple takes and use two or three of those to build the double tracks. Second, when I double track, I typically use it throughout the song. The article suggests that intermittent use of double tracking can also be effective. For, example double track only on the chorus and use a single track on the verses. The article also mentions double tracking the harmony part, which I do this as well.
The beauty of music creation is that there are so many different ways to get there. Take the path that makes the music you want to make.