Yukatas are for summer and what most people used to wear all the time before styles changed. They used to be normally just blue and white or such colors. Now Yukatas come in many colors. They are also much less formal, easier to put on (less layers and simpler obi), and less expensive than a Kimono. I have 2 Yukatas and each cost me about $300-400, the Obis about 100 (they are both nice) and the Geta shoes 80 (because they are special wood). Geta are usually half that for women (I also have big feet
).
For my friend's graduation from high school, her parents got her a Kimono, and it cost over $10,000 for the whole thing (obis, geta, hair, stockings, etc included, but the kimono itself was most of the $$$).
Also, Yukata is just the robe with a simple tie in back Obi and no stockings. You have small ties that go under the Yukata, but none really on top unless it's 'cool style' accessories, but not common.
Kimono has at least 2 layers, a much longer and more complicated obi, stockings, etc and also is always silk. The Yukata can be other fabrics, but mostly it is common to get cotton.
One thing I love about Kimonos and Yukatas are the purses that you can buy with them. In Kyoto, by Gion, you can see the most beautiful accessories for those things, because that area is famous for the Geisha.