What is difference between UniqueID and ClientID in ASP.Net Controls ?

Control.ClientID 

ClientID will be a unique ID string that is rendered to the client to identify the control in the output HTML. It uses _ to include parent controls (container) ID to make it unique. For example, ct100_ContentPlaceHolder1_txtLogin. The client id for control will be generated by appending the parent container's(parent control) ID with the original control id with "_" as delimeter. In our example, ContentPlaceHolder1 is the ContentPlaceHolder ID of the master page which is the parent container for txtLogin.

To get the ClientID of a control,
txtLogin.ClientID

The client id can be used to identify the control in client side scripting like javascript.
 
Control.UniqueID
UniqueID is also a uniquely identifiable ID for a control but only used by ASP.Net page framework to identify the control. It uses $ to include parent controls (container) ID to make it unique. For example, ct100$ContentPlaceHolder1$txtLogin. Similar to ClientID, UniqueID is also generated by appending the parent container's(parent control) ID with the original control id but with $ delimeter.

To get the UniqueID of a control,
txtLogin.UniqueID

 
The unique id is used by the Asp.Net framework to identify the control when the page is posted back to the server.