Perhaps the most traditional item of clothing is called a guayabera, which is a button-down shirt that is typically in white. This simple shirt can also be extended to be a dress in the same style for women. More stereotypically traditional is the bata cubana, which is a bright, colorful, loose-fitting, short-sleeved dress that reaches nearly to the ground. It often has ruffles and a large collar-like circle at the top surrounding the neck.
Today these traditional outfits are only popular at certain festivals and events, but can sometimes be seen elsewhere. Modern western-styled clothing is the norm in Cuba today and this dress can vary drastically in style and cut. In Havana and other cities the dress tends to be a bit more formal (unless people are going out dancing), while along the beaches the dress can be very liberal.
As a visitor to Cuba you can wear just about anything, but it is better to cover up a little if you are not on a beach. Although many people dress very liberally, there are a large number of conservative Catholics on the island who appreciate not seeing so much skin. Just try to dress for the occasion; a swimsuit on the beach is fine, but if eating in a restaurant, even one on the beach, try to dress a bit more modestly, and in churches long pants and shirts covering at least the shoulders are required. Lastly, and perhaps most importantly, avoid wearing anything that indicates, represents, or promote any political entity or even a religion as the government officials won't take kindly to this.