A motherboard is a Printed Circuit Board (PCB) that connects your processor, memory and all your expansion cards together to assemble a PC. Most motherboards made nowadays are ATX. An ATX motherboard has the standard I/O (Input/Output) connectors such as PS/2 ports, parallel ports, serial ports, etc, built onto the motherboard. Old AT motherboard on the other hand uses I/O cards and cables which needs to be plugged into the motherboard, which gets a bit untidy. AT motherboard requires AT keyboard and AT power supply. ATX motherboard fits into an ATX case and comes with an ATX power supply. The following is a pictures of an ATX motherboard.
As you have seen from the enlarged image, the motherboard comes with various expansion card slots and connectors. It comes with 3 different expansion slots, 1 AGP, 5 PCI and 1 ISA slot. The AGP (Accelerated Graphics Port) is where you would connect and AGP graphics card. The PCI slots is where you would connect cards such as sound card, modem, tv card etc. The ISA slot is quite an old type of bus which is handy if you got some old hardware such as an old ISA modem or sound card. The other connectors includes the Intel socket 370 CPU connector, the DIMM slot for SDRAM, IDE connector for connecting your HDD, CD-ROM or other IDE devices, and FDD connector