Here's a simple graphic I made to try and get my head around the Doctor's incarnations and their chronology.

A: First Doctor (William Hartnell)
B: Second Doctor (Patrick Troughton)
C: Third Doctor (Jon Pertwee)
D: Fourth Doctor (Tom Baker)
※: The Watcher (Adrian Gibbs)
E: Fifth Doctor (Peter Davison)
F: Sixth Doctor (Colin Baker)
G: Seventh Doctor (Sylvester McCoy)
H: Eighth Doctor (Paul McGann)
I: War Doctor (John Hurt)
J: Ninth Doctor (Christopher Eccleston)
K: Tenth Doctor, pre meta-crisis (David Tennant)
★: Meta Crisis Doctor (David Tennant)
L: Tenth Doctor, post meta-crisis (David Tennant)
☆: Valeyard (Michael Jayston)
M: Eleventh Doctor (Matt Smith)
*: Dream Lord (Toby Jones)
N: Twelfth Doctor (Peter Capaldi)