Bicyclic or polycyclic compounds are molecules that contain two or more rings that are joined together. There are three different ways that rings can be joined. The most common are fused rings, where two atoms and the bond between them are shared. When more than two atoms are shared, the result is a bridged system, so-called because the shared atoms form a "bridge". Finally, when two rings are joined at a single atom the result is a spiro linkage.
Ring fusion can occur either in a cis- or trans- fashion. Cis-fused rings are joined with bonds that are cis with respect to both rings, and trans- fused rings are joined with trans bonds. While cis fusion is possible with rings of all sizes, trans fusion occurs only on sufficiently large rings that are able to accommodate the increased distance between two trans bonds. For instance, trans-fused cyclopentanes are much less stable than trans-fused cyclohexanes, or trans-decalin.
The most stable conformation of both isomers of decalin is one in which both rings take on chair conformations. Drawing the cis isomer is a bit tricky because one of the chairs has to be drawn sideways. While the cis isomer can take on two conformations via simultaneous chair flips, the trans isomer is conformationally rigid. Neither ring of the trans isomer can undergo a chair flip because doing so would make the other ring impossibly strained.