The loop component is a kind of pseudo-component; It is only used within the various -list components (like fm:MediaList, fm:CommentList etc).
Basically, this is an iterator to process data returned by list components; for every item returned the loop will process it with the markup provided within it's tags.
Have a look at the following example:
<fm:CommentList pageSize="5">
<ol>
<fm:Loop>
<li>{$comment_comment}</li>
</fm:Loop>
</ol>
</fm:MediaList>
outputs:
<ol>
<li>Here's my comment!</li>
<li>Here's my comment!</li>
<li>Here's my comment!</li>
<li>Here's my comment!</li>
<li>Here's my comment!</li>
</ol>
This component does not accept any attributes.
This component inherits variables from its parent component; variables created by, for example, an fm:MediaList are used for every item returned by the Loop component.