The picture above is of 4 DIMM sockets. DIMM sockets are where the computer's RAM, (or Random Access Memory), is installed. DIMM stands for Dual Inline Memory Module. The reason it is called "Dual" is because both sides of the memory module have completely separate connections from the other side of the module. On the older SIMMs, (or Single Inline Memory Modules), both sides were connected together at each contact point on both sides of the module. Below is a partial picture of a memory module's little gold contact points.
These contacts are on both sides of the DIMMs and SIMMs. There are actually twice as many contact points on DIMMs as there are on SIMMs, because the contact points are not connected to the points on the other side of the module. On SIMMs the contact points are connected from one side to the other, which makes them the same connection for both sides.