MAIN Block
All sites have a main menu with the links to the most visited pages or important system areas. This block will appear as first one in the left of your site. You can't delete the Main Block and you just have one block.
This block is usefull to link to the different sections, like a Web Links, FAQ page, Virtual Store, Top10, Special Sections, Users Account Page, Topics, Statistics, etc. You can use HTML code but not PHP code inside the Admin Block.