What are the differences between Beowulf the movie and the poem?

Unfortunately, you would be better off asking what
similarities exist between the epic, Anglo-Saxon poem
Beowulf and movies that have been made supposedly telling the same
story.  Why?  Because the answer would be much shorter. 

If
you're referring to the latest theatrical version released (that I know of, anyway), the
animated version, a hero named Beowulf does kill Grendel.  That's pretty much where the
similarities end.

As far as the differences, here are some
of the main ones:

• Beowulf is not Grendel's
  father.  In the poem, Grendel is born of the cursed descendants of Cain and in
  slime.


• Grendel's mother does not look like Angelina
  Jolie.


• Grendel's mother is not a shape shifter. 
  


• Beowulf and Grendel's mother never have sex.  Grendel's
  mother is a monster the same as Grendel is. 
  

Unfortunately, movie makers have never made a
movie even remotely resembling the epic poem.  The story in the poem has been liberally
borrowed from numerous times.  The Thirteenth Warrior, for
instance, focuses on a warrior journeying to a troubled spot to aid the locals.  But
nothing thus far produced seems to even be an attempt to actually relate the story of
Beowulf and Grendel.