Are you talking about MicroATX or Mini-ITX? They're just motherboard sizes form factors. The major difference between the two:MicroATX
- Usually has 4 RAM slots
- Four expansion slots available (Typically three PCI-Express and one PCI if it has one)
- Usually has at least 2 6GBps SATA ports, 4 3GBps SATA ports
- Two RAM slots
- One PCI-Express x16 slot
- Usually 4 SATA ports (2 6Gbps and 2 3Gbps), may have 6
Otherwise, there's nothing else special about them. They take all the same CPU, RAM, video cards, etc. that ATX motherboards take (barring chipset and other compatibilities of course). The only thing you have to worry about is the case, which may be small. Most MicroATX cases are fine (except slim, desktops, and HTPC). Many Mini-ITX cases are not if you're doing heavy workloads. If you're going Mini-ITX with heavy workloads, I suggest something from SilverStone, Lian-Li, or Bit-Fenix.