Lolita is a fashion style which originated in Japan in the 1980s, which then began to grow and become popular in the 1990s. It was influenced by such people as designers like Mana and is growing more and more popular outside of Japan.
This fashion style is in no way related to Vladimir Nabokov's novel. Lolita is not sexual and does not involve young girls attempting to attract men older than themselves. I cannot put it any plainer than that.
There are the main substyles of lolita, including sweet, gothic, punk and classic. Some of the most popular brands of lolita clothing which centre around these main substyles are: Angelic Pretty, Baby The Stars Shine Bright, Metamorphose temps de fille, Moi-même-Moitié, and Innocent World. There are also many "offbrand" places to find lolita clothing. I believe that offbrand and brand are both good, and it's just a personal choice between the two. No one should be discriminated for having offbrand clothing.
There are also many other substyles, to learn about them click here. Many lolitas focus on one style and run with it, however you can find your own style of lolita and do what makes you feel comfortable and happy in your style!
To learn more about lolita, please click here! Sorry I can only give a basic definition since I am fairly new to lolita, but I hope to improve this post over time as I learn more.