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, USB ports, parallel port, serial port, etc, built onto the motherboard. ATX motherboard fits into a standard ATX case. The following is a picture of a Modern ATX motherboard.
As you can see from the image, the motherboard comes with various expansion card slots and connectors. It comes with 4 different expansion slots, 2 PCI and 2 PCI Express slot.
The PCI slot is where you would connect cards such as sound card, modem, tv card etc.
The PCI Express slot is where you would connect a PCI-E graphics card. This particular motherboard allows to have two different graphics cards in a SLI or Crossfire configuration.
The other connectors includes Intel LGA 1366 Socket, DDR3 Memory slots, ATX 2.0 24pin power connector. SATA connector for connecting your Hard drive, DVD-ROM etc.