While there isn't any sort of official biker uniform, many bikers do wear similar outfits. The stereotype is that bikers dress in leather and wear a lot of skull accessories, but that's not always true. Many do wear leather, but it's not because of the look (or at least not totally for the look). Leather is a very durable, protective material, and it won't fray or rip as easily as other fabrics if you go skidding across the hot pavement. It can help save your skin from getting severe road rash. However, jean material is also very durable, and so many riders choose to wear jeans and jean jackets.
Then there are the decorations. While many bikers do love to cover their riding gear with embroidered patches, some choose not to do this at all. Others, especially those who are part of a motorcycle club and are taking part in a club event, may wear a jacket featuring club patches. This is about as serious as the biker uniform gets: some clubs ask that members wear these jackets at events so that they can be identified. It's also a good way of advertising the club.
Biker jewelry does include bracelets, earrings, and necklaces with skulls, but there are many other decorations, too. Some jewelry, especially for men, is made to resemble bike chains and other parts. There are also pendants that look like bikes or that are shaped like eagles. Just about anything can be worn with your biker gear, especially if it matches some of your biker patches or other decorations.
The most important aspect of any biker outfit is protection, and that's why many bikers dress alike. Durable boots, tough leather clothing, gloves, and helmets are all ridden to help protect the biker in case of a crash.
