ergophobe - 4:59 pm on Jul 2, 2012 (gmt 0)

A space indicates that it is part of the cascade; no space does not

div.class3 refers to a div tag with a class attribute of "class3"

div .class3 should refer to a div tag that has a child of class3 and might be the same as, for example, div.class1 p.class3