Basically, the difference between a kimono and a hakama is the difference between a robe and pants. Of course, in Western culture (as far as I know) we don’t wear pants on top of our robes (although we might wear pants under a robe). So, there’s really no Western equivalent to use for comparison.
The kimono is a traditional full-length Japanese robe. Just as with Western robes, a belt is used to hold the kimono in place. However, when worn by women, the belts, called obi, are generally very wide. Many cover the entire abdomen. The obi are tied in the back rather than the front.
Sometimes confused with the kimono is the keikogi. The keikogi is the practice uniform worn by Japanese martial artists. It is usually composed of a robe-like shirt tied with a belt and a pair of pants drawn with a string at the waist. While a kimono might be used as a keikogi (I think some kendo’ka may use an actual kimono), they are not synonymous.
These days kimonos are mostly worn by women. Men typically wear kimonos only on special occasions nowadays. When men wear a kimono, they traditionally wear a hakama over it. The hakama is a pleated skirt or pair of pants, usually solid black, white or indigo blue in color. Hakama are also frequently worn by Japanese martial artists, particularly those linked to the traditions of the samurai.