A thesis studying the plasma structures encountered by the CLUSTER spacecraft along its orbit during 2001-2004 is presented. CLUSTER is the first space mission dedicated to the three-dimensional study of the terrestrial magnetosphere and its polar orbit and four spacecraft tetrahedron formation enables in situ measurements in various regions. The first part of the thesis describes and tests, two multipoint analysis tools, which include the spatial gradient and the time delay, using synthetic datasets. These analyses reveal the boundary motion and orientation, and the dimensions of the structures encountered, which are unable to be determined with a single satellite mission. The other part of the thesis deals with the plasmasphere, focusing particularly on the topology and dynamics of small- and large-scale density structures. The study also presents and discusses datasets from other CLUSTER instruments, and global images of plasmasphere.