Microsoft Paint (formerly Paintbrush, commonly referred to as MS Paint) is a raster graphics editor included in Windows 95 and onward.
Paint is commonly used on the AlternateHistory Forums to create maps and was used by early mappers due to its ease of use and inclusion in Windows. However, Microsoft Paint does not have layer support, transparency support or many other features commonly used to create mapping video slides. Thus, it does not see much usage in mapping videos today.
Read more on Microsoft Paint @ Wikipedia.