Lamorna, these guys have given you the basics and all quite correctly...personally i'd post this over in the recording forum...if you get an answer from sub, lwilliam, or jody i would say it is from the horses mouth, guys who live it and do it at a professional level. i layer tracks all the time but am not nearly as accomplished as i wish at the mixing of those layers. In todays competitive songwriting landscape, writers are having to become gifted in recording their works as well to be competitive. If i have a great layered recording of something i've written it is because i hired out the studio engineeing, i don't have the gear or skills in that department i wish i did. A properly layered song pitched appropriately to an artist in their key and style could be called the vehicle in which 90% of cuts ride in on.