This type of company management is very different from a traditional hierarchial bureacracy, where each employee reports to only one supervisor. The definition of Matrix Organsation as given in WIki:
Matrix management is a type of organizational management in which people with similar skills are pooled for work assignments. For example, all engineers may be in one engineering department and report to an engineering manager, but these same engineers may be assigned to different projects and report to a project manager while working on that project. Therefore, each engineer may have to work under several managers to get their job done.
and it can be picturised very well with this diagram...
picture credit here