You could place any HTML text to the Home, Products Listing or other pages by specifying the content in any custom block available in CMS > Custom Blocks section and then adding the block via CMS > Pages Layouts.
If you need to show custom block only for certain categories, manufactures, products etc. there is a number of Parameters you can use in custom block settings (CMS > Pages Layouts > click on 'tools' icon of the block):
item_id - for products, articles and ads
category_id - for any kind of categories
manf - for manufacturers
search_string - when a user is doing some search
article_id - to show block only for some articles
0 (top category), 13 and 48 are the categories where the block will be shown.