The Daleks appeared in the second ever Doctor Who story, and rocketed the series to immortality. Those early Daleks were crude, though. Powered by static electricity, they were unable to leave the metal floors of their city and could be disabled by being pushed onto a carpet (or a conveniently-placed cloak).
When they returned, they had been improved, with little radar dishes on their back to receive power being beamed to them so that they could Invade Earth. From then on, they seem to have developed the Generic Monster Robot Infinite Internal Power Source and there was no stopping them (except, of course, when the Doctor stopped them, which was every time). Yet when the Doctor travels to the original creation of the Daleks in Genesis of the Daleks, there is no sign of this earlier form.
See more at http://www.bbc.co.uk/dna/h2g2/A52005700