[]byte is a buffer. (io.)Reader is any type that can read bytes, like as part of a stream. There are `bytes.Reader` and `bufio.Reader` and `strings.Reader`, and

[]byte is a buffer. (io.)Reader is any type that can read bytes, like as part of a stream. There are `bytes.Reader` and `bufio.Reader` and `strings.Reader`, and many others, depending on what you're reading. These three types allow you to read []byte, any buffered input, and string, respectively, as streams. Streaming is more efficient for large pieces of data (files, etc; whatever you have), but the buffer is usually easier to work with and grants more flexibility.