Beowulf is a 2007 film adaptation of the Old English epic poem. It was directed by Robert Zemeckis, written by Neil Gaiman and Roger Avary, and starred Ray Winstone as the title character. Since every adaptation is interpretation, there will always be differences and changes. As Gaiman said, "The glory of Beowulf is that you are allowed to retell it." The first significant difference is that all the characters are speaking contemporary English, whereas the anonymous poem was written in Old English, a language incomprehensible to all but scholars. This makes it sound more modern and up to date than it is, even if the writers attempt to write in an "archaic" style.
The other major difference is that the filmmakers attempt to make the story into a fantasy adventure epic, much in the vein of Lord of the Rings or Harry Potter. As such, it has a much different feel than the poem, which conjures a shadowy, iron-clad world of honor, death, and monsters. Speaking of monsters, the look of the adversaries, Grendel and Grendel's Mother, were widely derided by critics. The film uses motion capture and was released in 3-D, making the monsters look incredibly cheesy and fake. Crispin Glover was cast as Grendel, and he looks more silly than scary, somewhat like Gollum. Angelia Jolie plays Grendel's mother, and though there's not a detailed description of her character in the original, the film makes her into a sultry, temptress-like figure.
A final difference is that the cultural context of the poem is almost totally lost. It was composed sometime in the 10th century, although the story had probably been around for much longer, and is set in Scandinavia. There is little cultural/historical specificity about the film, which turns the epic into a jumbled mess of garish swords and sorcery spectacle. If you want to look at the actual poem, I'd suggest the Seamus Heaney translation.