Regarding to asic,cpldand pla the i/o cells play a major role in designing a particular digital chip/incorporated digital device in a single chip.. So basing on that all digital devices or any device in the nature should have input as well as out put . For that if we have made a standerdised i/o cell which will results as a common and user frendly natured cell to give i/p or take o/p. This made to standerdise the i/o cells that has to be standerdly mantained, it should be followed by every manufacturers while designing or making their devices.

I/O cell is a interface between chip's core and outside of the chip, for example other chips integrated on one PCB. I/O cell or I/O buffer is a bidirectional buffer which can transmit and receive signals. moreover, I/O cell protect chips' core from high voltages, ESD, and avoiding gate oxide breakdown. 

To describe characterisation we can say that the Standard Cells have only one voltage source. I/O Cells may have multiple (usually two) voltage sources. The definitions of voltage sources are differentfor the two types of cells, and they have to be handled carefully.