When you have a iPhone and you download apps why is some of the apps I downloaded saying waiting and not downloading?

I don't exactly know why they do that, but I know how to continue the download. *Turn it off* then turn it back on. This should continue the download.

*If you don't know how to turn it off, here's how- hold the menu button + the sleep button for about 5 seconds, and a thing will pop up saying 'Slide to turn off'. Just slide it and in a few seconds it will turn off.