They are same, which denotes the parent stage

The property _root refers to the main timeline of the movie
from which the property was referenced, or the top-most
timeline within the current hierarchy of timelines.

The property _level0 references the main timeline of the
movie loaded into level 0. This property can be referenced
from any level and still reference the same timeline, that
on level 0.

Every level has a root inside it, like _level0 has a root 
timeline of its own, level1 has its own and so on..

Untechnically and in explanation point of view a root can 
be said as a child of level.

We use levels whenever we need to load new swf, jpg or gif 
files inside our current working swf but not in a movieclip 
and need to be placed above the current elements without 
replacing any.

Any swf loaded in at level0 will replace all the contents 
with the new but placed in level1 with bring every new 
content above the old.

So level0 refers to one root of his own and level1 refers 
to another root of his own. Every level has a root and then 
corresponding child movieclips and elements inside one 
another.

_root is the main movie clip on flash

but vritualy multiple _level holds multiple _root

eg:
_level0._root.mc_1
and
_level0.mc_1

refers one object