The walls of many older houses are covered with plaster rather than drywall. Plaster is applied as a wet paste over a series of thin wood strips called lath, which are attached to the wall studs. The plaster is squeezed between these thin pieces, oozing into the wall cavities. When the plaster dries, this ooze, called keys, holds the plaster to the wall.
More recently installed plaster uses an expanded metal mesh rather than wood lath. Use tin snips to cut through the metal mesh. The most difficult aspect of removing plaster is avoiding damage to adjacent areas.
Before you begin removing plaster, turn off any nearby circuits at the electrical service panel. Remove all receptacle and switch coverplates. Wear a dust mask rated for fine dust.
About 1 to 2 hours per 8-foot section of wall
Hammer, pry bar, power drill/driver (for attaching reinforcing 1x2s), handsaw, reciprocating saw (for removing parts of walls)
Prying, pulling nails, cutting with a reciprocating saw and handsaw
Isolate the work site to contain the mess; determine what utilities may be contained within the wall
1x2s to reinforce surrounding plaster, if necessary
If you remove only part of a wall, stop at a stud. You can't cut through lath in the middle of a bay without destroying the plaster. To find the end stud, drill 1/8-inch-diameter holes every inch through the waste section of plaster until the drill hits a stud. Attach a reinforcing 1x2 along the stud.