Stands for "Small Outline Dual In-Line Memory Module." Most desktop computers have plenty of space for RAM chips, so the size of the memory modules is not a concern. However, with laptops, the size of the memory modules matters significantly. Because laptops are designed to be as small and as light as possible, the size of each component matters. In fact, laptop parts are so crammed together, large RAM chips often do not fit into the overall laptop design. This is why SO-DIMMs were created.
A SO-DIMM is about half the length of a regular size DIMM. This allows greater flexibility in designing the memory slots for laptops. Many laptops have a user-accessible section that houses the SO-DIMMs, which make it easy to upgrade the computer's RAM. If the RAM chips were full size DIMMs, this type of design would be harder to incorporate and would likely increase the size of the laptop.
The first SO-DIMMs used 72 pins (or connectors) and supported 32-bit data transfers. Modern SO-DIMMs, however, typically use 144 pins, which allows for the same 64-bit transfers that a regular size DIMM does. While SO-DIMMs are used primarily in laptops, some desktop computers with small form factors also use SO-DIMMs to reduce the overall size of the case. However, when size is not an issue, regular DIMMs are typically used because they are a more cost-effective solution.