Brochures are the folded papers you usually see placed with the company information on it. Brochures tend to be most effective when giving details about the company rather than for a one-time event or party. They can be still be used for promoting a special event, however you could consider a flyer if that is what you had in mind. Detials about a particular department or a brand new service that you will be offering would be ideal to creat a brochure.
Letterheads are usually placed at the top of the business documents and contain the company logo and contact information.