Porcelain veneers are thin layers of porcelain attached to the outer surface of the tooth. Composite veneers are engineering resins adhered to the outer surface. Both are made to match your teeth (or whiten) and improve your overall smile. Veneers cover part of the teeth, but they are not the same as a crown, which is thicker and covers the entire tooth in 360 degrees: the front, the sides and the back.
Crowns will also require more tooth grinding, which isn't necessary with veneers. Porcelain veneers look more natural because the translucent properties allow them to capture light like a natural tooth. Composite resin still looks good, but it can't match the look that porcelain provides. Porcelain is more translucent than composite material; it reflects light in a similar way to natural tooth enamel.
In short, this material looks more like real teeth; however, this type of veneer is permanent and it takes more time and money to replace one if it's broken. When you consider the initial cost of porcelain and the fact that these veneers don't chip or break as easily as composite veneers, you could argue that the cost of porcelain equalizes over time. Talk to your dentist about your specific cosmetic goals and find out if dental veneers are a good option for you.