I. C. stands for Integrated Circuit. If you could look inside one with a magnifying glass you would see what looks like a downtown map. This is a pile of semi-conducting photographic images that form complex electronic circuits. Each IC can replace thousands of transistors. The contents of any one of those ICs in your computer would have filled a large room with 1942's finest technological equipment.
ICs can do many things.
Some are ROM chips, which stands for Read Only Memory. Memory of this sort is like a newspaper. You can read it as often as you like, but you can't change the news. Others are RAM chips. RAM means Random Access Memory.