<MARQUEE ...> creates a scrolling display. <MARQUEE ...> is an MSIE extension and will probably never be supported by any other browser. <MARQUEE ...> is often regarded as one of the "evil" tags, and that perception alone might be enough reason to not use it. However, used lightly and with taste (and understanding that it will never render everywhere), <MARQUEE ...> isn't such a bad tag. It can work well for announcements.
The basic use of <MARQUEE ...> is simple. Put most any kind of markup between <MARQUEE ...> and </MARQUEE>.