Does anyone else watch this? I recently started watching the new series, and its AMAZING.
Coolio, it gets a bit complicated, but the show's been on the air for almost 50 years with a break from 1989 to 2005, with only a TV movie during that break in 1996. As a result, 11 actors have played the Doctor over that time.
The Whoverse therefore dates each era by the actor who plays the Doctor, using a concept of "regeneration" during which the character of the Doctor changes his appearance. Thus, we have 11 Doctors (called in conversation "One" through "Eleven." who is the current incarnation.)
Then, the Doctor has one or more Companions who usually travel with him. The current Doctor, Eleven (played by Matt Smith), usually has had Amy and Rory (now husband and wife) traveling with him in series five and six; and series seven is being filmed as we speak.
There have been two previous incarnations of the Doctor since the series returned in 2005. Nine (played by Christopher Eccleston) was only seen in the first series after the return, David Tennant, who played the Tenth Doctor, was in series two, three, and four, and the specials that aired from Christmas 2005 to 2007 plus five specials beginning at Christmas 2008, and he left the series in a special that aired on New Years' Day 2010 in most of the English speaking countries that currently air Doctor Who. Ten had different companions -- Rose (who had traveled with the Ninth Doctor), then Martha, then Donna, and then others in most of the specials. Ten also had two Companions who had spin-off series, John Barrowman's Captain Jack, on Torchwood
and the late Elisabeth Sladen, as Sarah Jane Smith, who had originally traveled with the Third and Fourth Doctor in the original era of the series. (The Eleventh Doctor also appeared in an episode of the Sarah Jane Adventures
, making Sarah Jane the only companion to have traveled with four different incarnations of the Doctor (outside of anniversary and one-off specials). SJA ended as a result of Elisabeth's death in 2011 due to cancer.
So if we can figure out which incarnation of the Doctor you are watching, or which Companion(s) are with the Doctor, then we'll know where you are starting in with the series.