What is a motherboard? - Well the place to start with when building your own PC is the motherboard. This is the spinal column of the PC, the place that connects all of the other components together. These are PCB (Printed Circuit Boards) with components to connect the following:
The motherboard should be chosen to match the CPU that is designed to be fitted into the motherboard's CPU plate. You cannot fit an AMD processor into an Intel motherboard and vice-versa. Intel Motherboards are now the most popular with the i7 Intel processor being the fastest and most powerful
processor, and of course the most expensive.
The motherboard is fitted inside of a Computer Case and usually screwed in place (Dell PCs often use a single screw, where the board slots in using custom built slots into the motherboard. This means that you must buy the same Dell motherboard to fit into the case).
The motherboard has either SATA or IDE connectors for HDD/SSD and sometimes both. Care must be taken in motherboard selection to ensure that the correct interface is present for the desired Hard disk and CDROM/DVD drives.
New Power supplies often use the new black coloured power connectors used with SATA Devices. If you have an old drive and need the old Molex connector - then you will need convertor cables to connect these old devices. Also older PSUs used a wider motherboard connector which do not connect to modern motherboards.
Generally the rule is if you upgrade the motherboard, you need to also upgrade the CPU and PSU to ensure that all match.
Memory (RAM) - Also Motherboards are only compatible with a certain type of Memory or RAM. Care should be taken in motherboard selection that the correct memory is chosen to work with the motherboard.
Hopefully that answers the question What is a motherboard? - when building your own PC - the motherboard and CPU choice is the 1st step - good luck on your PC build!