Elements following a floated element will wrap around the floated element. If you do not want this to occur, you can apply the "clear" property to these following elements. The four options are "clear: left", "clear: right", "clear: both" or "clear: none".
The element is moved below the bottom outer edge of any left-floating boxes that resulted from elements earlier in the source document.
The element is moved below the bottom outer edge of any right-floating boxes that resulted from elements earlier in the source document.
The element is moved below all floating boxes of earlier elements in the source document.
Big John, from Position is Everything, has documented a rendering bug to do with Win/IE6 and its problems clearing floats - well worth reading!
Other Max Design articles and presentations