There's some major differences between the T.V. series and the books by Jeff Lindsay.
The first book called "Darkly Dreaming Dexter" is what the intro says that the series is based off of. That book losely follows the first season. After that hardly anything is the same.
Things that are the same:
Dexter gets Rita pregnant and marries her ( in the books he marries her then gets her pregnant, in the series it's reversed). Vince is Dexter's best man in the book and in the T.V. series( at least he starts off that way). Dexter kills an innocent person. Deb is transfered out of Vice into Homicide. Deb is seriously wounded ( book by a knife and season 4 by gunshot both wounds in the same area of her body). Dexter in good with children. Dexter puts a drop of blood of his victims on a glass slide. He usually forces his victims to confess to their guilt before killing them. Dexter's first human victim is his foster father's nurse that is overdoesing patients. Deb uses curse words a lot.
Things that are different:
LeGuerta is killed by Brian Moser at the end of the first book, she is alive still after season 4. Doakes is killed at the end of season 2 in the books he is alive, just missing his hands, feet and tongue. Deb knows that Dexter is a killer at the end of the first book, in the series she only knows that Brian was his brother. Cody and Ashtor are kidnapped by Lila in season 2 in the books they fight off people that try to mess with them. They know that Dexter's true nature in the books and they want him to teach them things because they are like him from a "rough" childhood. Rita's ex-husband Paul is in prison in the books, in the series he was killed in a fight in prison. Dexter has no emotions in the books, in the series rarely shows them but he has emotions. Dexter has hardly any respect for his fellow Miami cops in the books as far as their ability to catch him or solve murders.
Things that are similar:
Deb has a partner that is suspious of Dexter: Quinn in the series and Coulter in the books.
~ I haven't read "Dearly Devoted Dexter" book #2 and there's a few episodes from season 3 that I haven't seen yet so there's probably some things that I don't know and maybe some things that I have wrong...