For those confused by the new show
Believe it or not I came across at least two Doctor Who fans this week who thought that the 2005 show was a remake of the original 1963 version and that there were no Doctors before the one that met Rose (The doctor's 9th life).
Some of us apparently didn't notice he had doodled pictures of his previous incarnations in the episode Human Nature, or that the 9th Doctor was amazed at his own big ears, or that the 10th Doctor (David Tennant) ran into Sarah Jane (who knew him from his 3rd and 4rth life) in the episode School Reunion. Somehow they missed these things. Or they missed that the title of The Doctor's song in Christmas Invasion was Song for 10 (his 10th life).
All right, I think I understand. It's because here in America Doctor Who airs on The Scifi Channel which happens to be airing a remake of the show Battlestar Galactic.
Doctor Who is not a remake, it's a continuation from where the old show and then the 1996 movie left off.
This explanation may contain spoilers.
The Doctor is an alien from the planet Gallifrey. He has thirteen lives. He doesn't age (at least not physically, as we humans do). He's over nine hundred years old. If fatally wounded though and his body 'dies' he undergoes something called regeneration. He takes on a new form. That's how the show has kept going since 1963. The Doctor of (2005 series 1) is his 9th incarnation. The show is very, very old. Exactly forty-four years old as of twelve hours ago, actually. The current Doctor (David Tennant) is his tenth life.
The 9th Doctor gave his life for his human companion, Rose (Billie Piper). Rose gazed into the heart of the TARDIS (The Doctor's living ship) in order to save The Doctor and pretty much became possessed by the soul of the TARDIS. It's thanks to the power given to her by this possession that Rose resurrected their friend, Jack from the dead, rendering him immortal (basis for the show Torchwood). The power was too much for her so The Doctor kissed her, taking the power into himself, sacrificing his 9th life for her. The current Doctor is his 10th life.
Doctors number 1 through 6 lived and died within the 1963 show until 1989.
In 1996 in the made for TV movie the 7th Doctor got shot and was accidentally killed on the operating table by incompetent surgeons who didn't realize he had two hearts.
The 8th Doctor (played by Paul McGann) more than likely was 'killed' (regenerated) during the Time War and became Doctor 9. The Time War is what killed The Doctor's people (though we learn the deranged Time Lord, The Master lived). The Time War also killed many of their enemy, The Daleks.
And 9 died saving Rose. Same man, different form.
If you missed the episode where the 9th Doctor regenerated it was Parting of Ways, the final episode of series (season) 2 of the revived show, or season thirty-something if you're counting from the original show. The 10th Doctor's first full episode was Christmas Invasion (first episode of season / series 3 of the revived show) though other than in the tail end of Parting of Ways, the 10th Doctor did first appear in the Children of Need special for 2005.