Mage_Core_Block_Text in Magento
It inherints directly from Mage_Core_Block_Abstract and extends its base class only with three methods:
1. public function setText($text)
2. public function addText($text, $before=false) {}
3. public function getText()
The block only stores a text string (it could be plain or html of course), and during rendering phase, the block simply outputs the stored string.
You could set the string to a value or append / prepend the text to the existing string.