GMail uses a labeling system as oppose to the more traditional folder structure you may be familiar with.

Multiple labels can be attached to a single email, then it can be archieved so it is removed from the Inbox view, but is not deleted. A traditional email system will only allow an email to reside in a single folder. 

Google have put together a serious of instructions on how to create, edit and manage labels within the GMail Inbox.

Google Support: Create and Manage Labels