Every poly has its preferred applicator, typically a brush or cloth. Some polys also come in aerosol spray cans.
Brush-on polys work best on flat surfaces where it's important to build up a durable film. Brushes hold a lot of finish, so you can cover a wide area each time you load them up.
Wipe-on polys are best for contoured surfaces—crown molding and stair balusters—where brushing might create drips. Wipe-ons form thinner coats than brush-ons, so use them when wear isn't a concern.
Spray-on polys come in handy on hard-to reach surfaces, such as shutter louvers and chair spindles. Aerosol sprays require good technique to avoid drips, and extra prep time to protect surfaces from overspray. Their thin films aren't as tough as the ones you brush on.