Before today, I hadn't known the difference between yukata and kimono, but seeing this lovely image by morbuto in his Flickr album inspired me to google "yukata."
Here's some excerpts from Wikipedia:
"Yukata(浴衣,Yukata?) is a Japanese summer garment. People wearing yukata are a common sight at fireworksbon-odori festivals, and other summer events. The yukata is a casual form of kimono that is also frequently worn after bathing at traditional Japanese inns. Though their use is not limited to after-bath wear, displays, yukata literally means bath(ing) clothes."
"Fabrics and Styles
A yukata is a cooling garment to wear. Like other forms of clothing
based on traditional Japanese garments, it is made with straight seams
and wide sleeves. Unlike formal kimono, yukata are typically made of cotton rather than silk or synthetic fabric, and they are unlined.
Traditionally yukata were mostly made of indigo-dyed cotton but
today a wide variety of colors and designs is available. Like the more
formal kimono, the general rule is the younger the person, the brighter
the color and bolder the pattern. A child might wear a multicolored
print and a young woman, a floral print, while an older woman would
confine herself to a traditional dark blue with geometric patterns.
Since the late 1990s, yukata have experienced a bit of a revival, and
many young women now wear them in summer in personally distinctive ways
not limited by tradition. This garment is very traditional."