"Ubiquitin is a small (8.5 kDa) regulatory protein that has been found in almost all tissues (ubiquitously) of eukaryotic organisms. Ubiquitination is a post translational modification (an addition to a protein after it has been made) where ubiquitin is attached to a substrate protein." 
http://en.wikipedia.org/wiki/Ubiquitin 
The addition of ubiquitin to proteins can signal for their degradation via the proteasome. 
This model is based on Human erythrocytic ubiquitin. The C-terminal glycine residue, conjugated to the lysine residues of target proteins, is represented as spheres. Model based on the Protein Data Bank entry 1ubq: 
http://www.rcsb.org/pdb/explore/explore.do?structureId=1ubq 
Model created using VMD: 
http://www.ks.uiuc.edu/Research/vmd/