Its is not available on premises. It's basically a chat-centered workspace in Office 365. It's not available as a standalone on prem product.
However, you can install the client application on your machine or phone. After you install and open it, it will ask for your work account or Microsoft account.
So in case of OnPrem users, you won't have work account. However, you can use the personal(Microsoft) account. So any user with the appropriate cloud license and who is in AAD(Azure Active directory) can login to Microsoft Teams, see other users' presence, send messages within Microsoft Teams, etc.
Reference - Introducing Microsoft Teams—the chat-based workspace in Office 365
Download link - Microsoft Teams download