In the TV show: Norman Reedus made a big impression on the show's producers when he auditioned for the role of Merle. In fact they liked Reedus so much that they wanted to make sure to include him in the project, even when the role of Merle ultimately went to Michael Rooker - so they created an entirely new character, just for him.
In the comic: Amazingly, Daryl and his brother Merle, two of he show's most memorable and iconic characters, are both completely absent from the comic books.
2. Merle's Not In the Comic
In the TV show: Series creator Robert Kirkman thought it was important for the TV show to include a classic, old skool villain who could act as a foil for Rick and the rest of his posse. We're extremely happy Mr. Kirkman felt this way, because Merle (played by legendary character actor Michael Rooker) turned out to be a villain of superb caliber. The Governor eventually shoots him in Season 3 as punishment for his disloyalty, and in an emotional scene, his younger brother Daryl takes out his reanimated corpse.
In the comic: Merle was an invention of the show writers, and doesn't exist in the comics at all, which is kind of a shame considering how really badass that DIY, weaponized knife-hand thing of his was.
3. Glenn Becomes a Farm-Hand
In the show: Glenn sticks close by Rick and the rest of his original crew diligently, and is instrumental in helping other members of the group relocate to safety during the prison raid.
In the comic: After Hershel's barn burns down in Season 2, Hershel kicks the rest of Rick's group off his property, but allows Glenn to stay and help out on the farm because of his relationship with Maggie. He catches up with the rest of the group at the prison later on, just in time to make himself useful.
4. Everybody Loves Morgan
In the show: Morgan's story, like that of so many other characters, is a tragic one. Morgan and Duane are the first living people Rick encounters after his bleary-eyed escape from the hospital, and Morgan's explanations about the recent, bewildering end of western civilization are punctuated by his painful inability to accept the recent death of his wife. After they initially part ways, Morgan pops up again in Season 3, having also tragically lost his son. He's paranoid, unstable, nihilistically enraged, and obsessed with setting booby traps.
In the comic: Morgan maintains a calm, sweet demeanor, and acts as a vital tour guide for Rick after he first regains consciousness. Also, though Duane does eventually buy the big one, Morgan's sad, creepy zombie wife isn't wandering around outside their house in the comic books.