While the basic appearance of Daleks remained the same throughout their history, there were several changes and alterations to Davros' original design of the Mark Three travel machine. Variants of the "basic" Dalek design were produced both by Davros and the Dalek Emperor/Dalek Supreme (these last two representing the major power holders in the Dalek hierarchy). Both factions of Dalek used a Dalek drone for basic combat.
A Dalek variant should not be confused with a Dalek using a different Manipulator arm, as these are not considered true variants.
Early in his design work Davros created Spider Daleks, which walked on a set of spider-like legs, rather than gliding across the ground. These were succeeded by Daleks known as 'Strider Daleks'. (PROSE: War of the Daleks)
Following his awakening from cryogenics and an encounter with the Dalek Supreme, Davros set up a base of operations on Necros and began creating Daleks of human origin. As the name suggests, these Dalek variants were created using humans, rather than Kaled mutants. This process also employed a Glass Dalek. (TV: Revelation of the Daleks)
Dalek Emperor / Dalek Supreme variants
Both Davros and the Dalek Emperor were known to use Special Weapons Dalek variants in combat situations: Davros used one during the Shoreditch Incident, whilst the Dalek Emperor utilised several in various campaigns. (TV: Remembrance of the Daleks, COMIC: Fire and Brimstone, PROSE: War of the Daleks)
Post-Last Great Time War
Logic dictates that the bronze Daleks would be the 'Mark IV', and the Progenitor Daleks the 'Mark V'. Following the Last Great Time War several new Dalek variants were created, both embryos, not technically a variant, but rather an attempt to recreate the original Kaled mutation. (TV: Daleks in Manhattan)
After the apparent eradication of the Dalek species as a whole, (TV: Journey's End) a small, ragtag group of Daleks found themselves stranded in the Second World War era. They concocted a plan to have the Eleventh Doctor identify them as Daleks to the Progenitor, an ancient Dalek instrument containing samples of the original, long-lost, Kaled DNA. The Progenitor created five new specimens, with five larger and colour-coded cases, named "Scientist", "Strategist", "Drone", "Eternal" and "the Supreme". The New Paradigm Daleks proceeded to destroy their 'degenerate' descendants/forebears, and fled to parts unknown through a time corridor to start anew a restored Dalek legacy after the Doctor, with the help of Amy Pond and Winston Churchill, defeated their plan. (TV: Victory of the Daleks)