These terms are sometimes used interchangeably, but it's pretty well established in the tech writing business that there's a difference. It's not that one is better than the other, but, generally speaking, they have different uses.
Flyer (also leaflet and handbill)
A flyer is a single sheet of paper, usually 8 1/2 x 11 (A4), used to get information to a large number of people inexpensively. Flyers are popular for small scale advertising.
They're often, quite literally, throw-aways because they're handed out at random or posted in public places in the hope they'll be read. If they're intended to be posted, such as on a bulletin board or put up in a window, they're printed on one side only. If they're going to be hand held or mailed, they can be printed on both sides.
The purpose of a flyer is to provide a limited amount of information for a limited time at relatively low production costs. Some common uses are:
- announcements of social or commercial events
- product information such as a spec sheet left in new cars
- fact sheets distributed at a press conference
While desktop publishing has made it easy for non-professionals to produce quite sophisticated flyers, many businesses rely on professional writers and graphic artists for the copy and layout of business flyers.
There is some disagreement on whether a flyer and a flier are the same thing. The preferred usage in Standard Written English is that a flier is someone or something that flies.
Brochure (also pamphlet)
Brochures are normally more sophisticated than flyers and more expensive to produce. As much time, effort, and expense can go into producing a quality brochure as producing a short video. Businesses produce millions of brochures every year, and a skilled brochure writer is well paid.
Brochures are used as sales or information resources that may be retained and read a number of times by a specific audience. They're commonly displayed in racks at airports, hotels, and doctors' offices, but as random as this seems, it's expected that only the people interested in the subject of the brochure will take one.
They're not intended to be throw-aways. They're widely used as follow-ups to initial sales contacts and as supplementary information sources. An insurance company, for example, might use one brochure as a sell-piece and later mail a different brochure to provide details of its policies to a person who has indicated interest.
There is no limit to the size or shape of a brochure, but the most common format is a single sheet of paper folded once - to create four panels - or twice - to create six panels. The first format is most often called one-fold or bi-fold. The second one is usually called tri-fold or tri-panel.