What's the difference between UTF8/UTF16 and Base64 in terms of encoding

In. c#

We can use below classes to do encoding:

• System.Text.Encoding.UTF8
• System.Text.Encoding.UTF16
• System.Text.Encoding.ASCII

Why there is no System.Text.Encoding.Base64?

We can only use Convert.From(To)Base64String method, what's special of base64?

Can I say base64 is the same encoding method as UTF-8? Or UTF-8 is one of base64?