The Daleks are a fictional extraterrestrial race of mutants from the British science fiction television series Dr Who. The mutated remains of the Kaled people of the planet Skaro, they travel around in tank-like mechanical casings, and are a race bent on universal conquest and destruction.
They are also, collectively, the greatest alien adversaries of the Time Lord known as the Doctor, having evolved over the course of the series from a weak race to monsters capable of destroying even the Time Lords and achieving control of the universe.
As early as one year after first appearing on Doctor Who, the Daleks had become popular enough to be recognized even by non-viewers. In December 1964 editorial cartoonist Leslie Gilbert Illingworth published a cartoon in the Daily Mail captioned "THE DEGAULLEK", caricaturing French President Charles de Gaulle arriving at a NATO meeting as a Dalek with de Gaulle's prominent nose.
Manufacturing the props was expensive. In scenes where many Daleks had to appear, some of them would be represented by wooden replicas (Destiny of the Daleks) or life-size photographic enlargements in the early black-and-white episodes (The Daleks, The Dalek Invasion of Earth and The Power of the Daleks).
In stories involving armies of Daleks, the BBC effects team even turned to using commercially available toy Daleks, manufactured by Louis Marx & Co and Herts Plastic Moulders Ltd. Examples of this can be observed in the serials The Power of the Daleks, The Evil of the Daleks, and Planet of the Daleks.
The first Dalek toys were released in 1965 as part of the "Dalekmania" craze. These included battery-operated, friction drive and "Rolykins" Daleks from Louis Marx & Co., as well as models from Cherilea, Herts Plastic Moulders Ltd and Cowan, de Groot Ltd, and "Bendy" Daleks made by Newfeld Ltd. At the height of the Daleks' popularity, in addition to toy replicas, there were Dalek board games and activity sets, slide projectors for children and even Dalek playsuits made from PVC. Collectible cards, stickers, toy guns, music singles, punching bags and many other items were also produced in this period.
Dalek toys released in the 1970s included a new version of Louis Marx's battery-operated Dalek (1974), a "talking Dalek" from Palitoy (1975) and a Dalek board game (1975) and Dalek action figure (1977), both from Denys Fisher. From 1988 to 2002, Dapol released a line of Dalek toys in conjunction with its Doctor Who action figure series.
Source and more history in this Wikipedia article.
What's it worth? Take a look at this Dalek price guide: sold listings for a value indication.