He is a Magrathean, and a designer of planets, most notably the fjords found on the coast of Norway on planet Earth. Slartibartfast won an award for his coastal design. When Arthur Dent and Ford Prefect were on ancient Earth, they saw this award (and Slartibartfast's signature) deep inside a glacier in ancient Norway.
When Earth II was being made, Slartibartfast was assigned to the continent of Africa. He was unhappy about this, because he wanted to make more fjords, and fjords in Africa would be hard for him to explain without natural glacial movement.
In the event the new Earth was not required, and much to Slartibartfast's disgust its owners suggested he take a quick skiing holiday on his glaciers before dismantling it.
Douglas Adams writes in the notes accompanying the published volume of original radio scripts that he wanted Slartibartfast's name to sound very rude, but still actually broadcastable. His first attempt was "Phartiphukborlz".
wikipedia.org dumped 2003-03-17 with terodump