Fullmetal Alchemist is often cited as one of the greatest anime of all time, but it's not always clear which series people are referencing. There are two versions of the anime - Fullmetal Alchemist, which came out in 2003, and Fullmetal Alchemist: Brotherhood, which was released in 2009.
There are some major differences between the two Fullmetal Alchemist anime. For starters, the 2009 series is far more faithful to the source manga, since the written version did not conclude until mid-2010. However, this gave the 2003 series a chance to take some interesting creative liberties, like turning Ed and Al's mother into a homunculus.
There's much debate over which series is better, and there's no objective answer, since both versions excel at different things. Even if you're a diehard fan who refuses to watch both shows, it's important to recognize the differences between the two anime, as each has qualities that merit celebration.
In FMA 2003, the homunculi resemble the human relations of loved ones who tried to reincarnate them. Since the homunculi are created by one figure in FMA Brotherhood, many of them have no relation to the show's human characters.
In Brotherhood, Sloth is an enormous, muscular man who embodies the sin for which he is named. While Sloth is not lazy in the 2003 anime, she's apathetic and emotionless. She was created in Ed and Al's failed attempt at human transmutation, and she resembles their mother as a result.
King Bradley is called Wrath in Brotherhood, but in FMA 2003, he's referred to as Pride. Wrath and Pride play crucial roles in both series, and new characters are introduced to fill the role not given to Bradley.
In the 2003 series, Wrath is an entirely new character who is born when Izumi Curtis fails to bring her son back. In Brotherhood, Pride takes the form of a young boy named Selim Bradley. This character does not appear in the earlier TV series, though he does show up in the manga.
While Envy is remarkably similar in both versions, his backstory is a little more interesting in the 2003 anime. According to the first version, Envy was created from a failed transmutation of Dante and Hohenheim's child, which means he's Ed and Al's half-sibling.
In both series, Hohenheim is Edward and Alphonse Elric's father, but his role in the story differs greatly. In the 2003 anime, Hohenheim is something of a minor player. Like Dante, he is an ordinary human who has managed to surpass his normal lifespan by creating a Philosopher's Stone and transferring his consciousness into another body. Dante and Hohenheim are lovers, but Hohenheim leaves her when he decides he can no longer pursue immortality at the expense of other people.
In Brotherhood, Hohenheim is given a much more central part in the narrative. Though he is originally enslaved, Hohenheim is granted nearly unlimited power by Father. At the end of the series, he faces off against Father and ultimately perishes after exhausting the power of his Philosopher's Stone.
The 2003 anime is slower-paced and more character-driven than the action-oriented 2009 series. While both shows do a good job of developing characters and interspersing moments of calm with intense action scenes, FMA 2003 is best described as a drama.
On the other hand, the 2009 anime is more of a classic shonen series.
Ethical issues play a major role in both versions of FMA, but each series has a different thematic focus. The 2003 anime explores the limits of science and questions how humans can ethically engage in scientific research. The story spends more time on Shou Tucker's gruesome experiments and details how homunculi are created from failed attempts to bring back those who have passed.
In FMA Brotherhood, the main themes are the inherent value of human life and breaking free from cycles of vengeance. These ideas are investigated through the story of the Ishvalan conflict, which occurred in an effort to create a Philosopher's Stone.