Size and socks are closely related in that the largest intel motherboard series on the market is designed specifically to host multiple Xeon-server class chips, while the tiniest ITX Intel motherboard series is only used for mobile CPU sockets or even Netbook/Nettop CPU sockets for Intel's ATOM family of budget CPUs. The middle two groups, ATX and Micro ATX, are both home to desktop CPU sockets, though there are a few server ATX motherboards, and at least one Micro ATX Intel motherboard featuring a socket designed for use with laptop CPUs. Obviously the larger the motherboard physically is, the more features it can integrate.
One of those features is memory sockets, which are becoming more and more important as 64-bit CPUs and 64-bit software continue to gain traction in the market. One of the main benefits of going 64-bit is the ability to address large amounts of system memory, but all that memory needs a place to plug into any Intel motherboard. This is where memory sockets come in, and larger Intel motherboards tend to have more of these than smaller ones because they tend to eat up relatively large amounts of physical space. Because most Intel motherboard designs support dual- or even triple-channel memory configurations, many memory sockets are found in pairs or in trios. The memory sockets are usually designed for very specific types of memory that operate within fairly limited tolerances when it comes to speed (MHz) and electrical requirements.
Memory and CPU sockets are not the only things that can be plugged into most Intel motherboards. All but the very smallest Intel motherboard designs feature slots designed to accommodate various add-in cards ranging from graphics adapters to high-end RAID array devices. These are generally referred to as 'slots' and come in AGP, ISA, PCI, PCI-X, PCIE, PCIE-G, and many other styles and types. Some types such as AGP are only useful for specific types of add-in modules, while other slots such as PCIE all general purpose and allow any Intel motherboard to be expanded and customized to perform any number of customized tasks.
Of course, many of those very tasks that once were exclusively handled by add-in boards are now part of the standard array of integrated features. Onboard sound, networking, USB controllers, SATA, and even advanced RAID controllers are all very common any almost every single Intel motherboard on the market. While some of these features such as RAID controllers come in many flavors, the general rule is that one gets what they pay for; a more expensive Intel motherboard will be more likely to have a more powerful integrated RAID controller. Some of these integrated features may work well for today, but having expansion slots helps to ensure that future upgradability and expansion without complete system replacement is possible if any portion of the integrated feature set is ever deemed insufficient.
