Yellowcake is a solid uranium-oxygen compound (U3O8) that takes its name from its color and texture. It is a product of the uranium milling process, essentially a half-way step between raw ore finished product. It is the material used for fuel enrichment and fuel pellet fabrication for nuclear reactors.
The raw ore is crushed and then washed with sulfuric acid, mixed with ammonia gas and then precipitated. The floury precipitate thus collected is yellowcake.
To obtain the valuable uranium-235 from yellowcake, one must separate it from the non-fissionable isotopes uranium-238 (over 99% of all natural uranium) and uranium-234 (less than 0.01%).
Material from several sources including
http://www.nrc.gov/NRC/EDUCATE/GLOSSARY/Yellowcake.html
and
http://chemistry.about.com/science/chemistry/library/weekly/aa061200a.htm.