Yukata is a simple cotton kimono without lining. The name originates from the name Yukatabira; yu = bath, katabira = underclothing. Traditionally in the Heian era the court nobles wore yukata after a bath, and this became more popular among the commoners with along the public baths. It is very popular still, because of it's simplicity and lightweight fabrics, that at comfortable in the hot, moist summers.
It traditionally has white and dark blue patterns that can vary a great deal. Nowadays modern yukata are very colorful and has more detailed patterns. It can be worn by everyone, men women, children and it can be worn at festivals, at home relaxing and as sleeping clothes. It is widely regarded as a casual a traditional blue yukata summer kimono, but is still used after baths, especially at the hot onsen springs, where each resort provides yukata in it's own pattern.
The yukata can be worn with a thin cotton sash matching the kimono when wearing for night attire. I public, a wider belt used, and usually the yukata is worn with a "hanhaba obi", which is an obi that are half the with of normal obis, tied in a cute butterfly or "clams mouth" bow. It is easy to wear, and even the obi can be tied unassisted, with a little practice.
a more modern, bright colored yukata
Both yukatas are from www.ichiroya.com