Points are a real world measurement originally used in printing/typesetting. A point is 1/72 of an inch, which means that precisely how large it is rendered on screen depends on the size of a logical inch on that particular computer. For instance a PC will typically use 96DPI (96 pixels per inch), meaning a pixel will appear smaller than a point, however Macs usually use 72DPI, so a point and a pixel would appear the same size to a Mac user.
jokoon: The <font> tag's size values 1-7 aren't real measurements, they're simply relative sizes based on the HTML document's base font. The actual sizes used and the difference in relative size between the values varies between implementations of browsers. For instance Internet Explorer displays fonts sized using <font size="some number"> much larger than Mozilla does. There's really not anything mIRC can do to use HTML browser sizes.