Sweet Lolita is one of the three main Lolita styles (the other being Gothic and Classic). It is charecterized by being the sugary cute princess style of Lolita. Pastel colors are the most common, such as pink, sax, mint, lavender and yellow. But the color does not make the style and there is also a lot of dresses released in colorways that are black, brown and red. The theme and styling is a better tell than the colors. Overall a Sweet Lolita's look should be like a princess or/and doll. Prints are very popular in Sweet Lolita and popular themes include but are not limited to: carousel horses, candy, berries, chocolate, cake, toys and stars. The make-up should be cute and false lashes and circle lenses are very popular. Pastel wigs used to be really popular but having your own hair is gaining popularity.
The biggest Sweet Lolita brands are Angelic Pretty, Baby the Stars Shine Bright and Metamorphose. They all have a distinct style and a lot of girls have a favorite that the stick to. A notable Sweet Lolita model is Misako Aoki who is one of the people organizing the Kawaii Ambassador initiative.
I made a collage to help you get the general feel of the style visually:
None of these are any rules of the style, just descriptions of whats common so that you can get an idea of what the style looks like, so don't be afraid to experiment!