ID vs UniqueID vs ClientID vs UniqueClientID vs StaticClientID?

Okay, I'm pretty confused about the IDs of dynamically created controls.

Public Class TestClass
    Inherits Panel
    Implements INamingContainer

    Function TestClassInit() Handles Me.Init

        Dim pnlMainPanel As New Panel
        Me.Controls.Add(pnlMainPanel)

        Dim pnlTest1 As New Panel
        pnlMainPanel.Controls.Add(pnlTest1)

        pnlTest1.ClientIDMode = UI.ClientIDMode.Inherit ' DEFAULT
        'pnlTest1.ID = "ctl01"
        'pnlTest1.UniqueID = "ctl00$MainPanel$ctl01"
        'pnlTest1.ClientID = "MainPanel_ctl01"
        'pnlTest1.UniqueClientID = "ctl00_MainPanel_ctl01"
        'pnlTest1.StaticClientID = ""

        pnlTest1.ClientIDMode = UI.ClientIDMode.Predictable
        'pnlTest1.ClientID = "MainPanel_ctl01" (no change)

        pnlTest1.ClientIDMode = UI.ClientIDMode.AutoID
        'pnlTest1.ClientID = "ctl00_MainPanel_ctl01"

        pnlTest1.ClientIDMode = UI.ClientIDMode.Static
        'pnlTest1.ClientID = ""

    End Function
End Class

Why the 5 different IDs??

When should you use the different ID modes?

(I read the MSDN docs, but they were, as usual, not particularly illuminating.)

If I don't care what the ID is, and just want to add a control & give its ID to a dynamically added AJAX extender, which mode/ID combo should I use?