Ribulose is a trivial name for the ketopentose (five carbon containing ketose) erythro-2-pentulose, usually found in nature as the D enantiomer, shown below in the furanose form (beware horrid ASCII graphics):-

   /    \  CH2OH
 /        \|
C          C
 \        /|
  \      / OH
   |    |
   OH   OH

A derivative of ribulose, D-ribulose 1,5-bisphosphate (RBP or RuBP) is extrememly important biologically as a metabolite in the reductive pentose phosphate cycle in photosynthesis, where it is a substrate of the enzyme Rubisco.