Blocks is a new concept introduced in version 7 of EPiServer CMS. Like pages which are instances of a page type blocks are instances of a block type. Unlike pages block types can be used in two different ways, as "shared blocks" and as properties on other block or page types.
While stored in the same tree structure as pages, the content tree, blocks aren't ment to be used as stand alone content. Instead both shared blocks and "local blocks" are ment to be used as partial content rendered as parts pages.