This page lists any existing volumes created using your EC2 accounts, and any snapshots of those volumes. You can create a volume using the New volume details form, which can then be attached to a running EC2 instance. Volumes that are no longer needed can be selected from the list and remove with the Delete Selected Volumes button.
A snapshot is a copy of a volume that is stored in S3, and can be used to create new volumes. Once you have at least one volume, this page can be used to create snapshots from it, and to remove existing snapshots.
Be aware that Amazon charges for volumes based on their size, amount of time they exist, and level of disk IO. Snapshots are charged at regular S3 storage prices.