Several aspects of the Bénard instability are approached in this work. After having described the principle of the experiments by underlining the role of the two driving forces of instability, namely surface tension and buoyancy, we show first of all, for configurations of large horizontal extension, various types of convective patterns, all made of 2D pavements of convective cells (hexagonal, square). The narrow similarity between these 2D patterns and the 2D polycristals (presence of grains, dislocations, grain boundaries) is shown. Structural order may be studied with the traditional tools (optical or mathematical Fourier transform, radial and angular correlation functions). A comparison between this (partial) disorder and mathematical models built independently of the physical phenomenon is established (minimal spanning tree, Aboav-Weaire, Lewis laws...) The agreement is surprisingly good. A study of the structural disorder can be presented as a function of Marangoni number until reaching spatio-temporal chaos. Dimensions of the attractors are calculated