The purpose of this tag is to define what keywords apply to your page. However, only some search engines will read this tag, and those that do often do not place as great importance on it as keywords found in other areas of the page. Still, you should generally include a META keyword tag on your page within the <HEAD> area.
Example of a META keyword tag:
<META name="keywords" content="blue widgets,green widgets,red widgets, Jerry's Widget Emporium">
You can choose to use commas or not to separate each keyword or keyword phrase in a META keyword tag. You can also use spaces to separate the words, or even commas and spaces.
Since none of the major engines are case sensitive, you are safe to list keywords in lower case only. Avoid repeating keywords more than two or three times, and never list the same keyword twice in a row.