Unique and non unique index in SQL server

A developer asked me a question yesterday about what's the difference of unique and non unique index in a DB table. I then google and redirect him to this short and clean article: http://www.w3schools.com/sql/sql_create.asp 

It's good to recall:

Updating a table containing indexes takes more time than updating a table without, this is because the indexes also need an update. So, it is a good idea to create indexes only on columns that are often used for a search.