what is the difference between a Kimono and a Yukata?

Seriously? Information on the two is all over the web, including Wikipedia. Not to mention that this question has been on Yahoo before (I know, I've answered it many times).

Also it has nothing to do with gender. Kimono and yukata exist for both men and women.

Yukata are made of cotton, are not lined, and are only appropriate for use during the summer. People usually wear them to festivals. They don't require as many accessories as a kimono (though some of the styles for women these days are very fancy and involve a lot of accessories -- these accessories are for fun and aren't functional the way kimono accessories are). They are EXTREMELY casual -- similar to a T-shirt and shorts. Yukata can also be worn as bathrobes at hot springs, and in fact that was their official function and these days the bathrobe ones and the festival/summer wear ones can look quite different in terms of the colors and designs used for them.

Kimono are generally made of other materials. Silk is very common, but wool and polyester are too. Depending on when they're meant to be worn they can be lined or unlined, and those made for summer wear are generally also made of very light-weight material. ALL kimono are more formal than yukata. ALL women's kimono require a juban, obi, obi age, obi jime, and usually a han eri to wear properly (men's accessories are slightly different). The patterns on kimono are generally season-specific, e.g. a kimono meant to be worn in winter will have winter-related designs on it, in addition to being lined and made of a heavier fabric. Different kinds of kimono exist for different occasions, from the fairly casual (komon for women) to the extremely formal (tomesode and furisode for women) and for special occasions like weddings and graduations. A yukata would NEVER be appropriate for these situations because of how informal it is.

A yukata is a type of kimono. It's similar to western casual wear such as a sport suit or casual suit as oppose to formal business suit. Yukata are considered very informal and casual whereas a kimono is worn for more formal occasions. In some cases, the yukata can even be very refine and expensive so that it can be confused for the more formal kimono..

Originally, the yukata was worn when relaxing or going to/from bath houses. The traditional Japanese inns (ryokan) and onsens would give its guest to wear on or near the premise almost like a dress code and souvenir.

It is only in recent times that the yukata has become summer wear and worn during festivals.