When you sign-up for the Power BI service at PowerBI.com (this address redirects to App.PowerBI.com), use establish a tenant for your organization, hosted in the Azure cloud. Even if you setup a 90-day trial account, you have a tenant that you can upgrade later on. By default, all of the datasets, reports and other assets you publish to the workspaces in your tenant, are in a shared capacity. This means that Microsoft will manage the storage and resource allocation among shared servers in their data centers. Larger organizations will benefit from a dedicated capacity “Premium” tenant. This means that Microsoft dedicates at least one virtual machine in the Azure data center, with guaranteed capacity, for your tenant. With Premium-per-User (PPU) licensing, you pay per user.
Click through for a list of available options and some of the implications of each.