here i can give brief understanding regarding magento block xml tag
name = Name of the block. It should be unique in the page.
*as = Alias *. Smaller form of name. It should be unique in it's parent block.
template = The template file (View) this block is attached to.
You can call methods from block type inside this by using $this.. e.g.
name vs. as example:
<block type="block/blocktype1" name="first_block" template="template1.phtml">
<block type="abc/abc" name="ty1" as="common" template="abc.phtml"/>
<block type="block/blocktype1" name="second_block" template="template2.phtml">
<block type="xyz/xyz" name="ty2" as="common" template="xyz.phtml"/>
So, we can now call block name ty1 from first_block AND ty2 from
$this->getChildHtml('common');, but see both the blocks called will be different as per their calling parent.