Two-Tier or Three Tier Architecture in SharePoint

If SharePoint Server and the database server are installed on one computer, it is a single-tier deployment.
In a two-tier deployment, SharePoint Server components and the database are installed on separate servers. In a three-tier deployment, the front-end Web servers are on the first tier, the application servers are on the second tier, which is known as the application tier, and the database server is located on the third tier.