Expanding sleeves are available in both plastic and metal. Driven with a hammer into a previously drilled undersized hole, they work by spreading against the sides of the hole when a screw is threaded into it. After the sleeve is installed into the wall, the screw is removed and used for hanging the item.
Molly bolts also use an expanding technique. Molly bolts with points can be hammered directly into the wall. Non-pointed molly bolts need a predrilled installation hole. The molly is seated into the hole using a hammer against the screw head. Once the flange is flush to the wall, the screw is turned, causing the slotted sleeve behind the wall to balloon out. The screw is then removed and used to hang the item.
Toggle wall anchors have prongs that use springs to expand inside the hollow wall. A predrilled hole, large enough to insert the anchor, is required. One the anchor is inserted into the wall, the screw is turned, forcing the prongs to tighten against the back of the wall. Once installed, the screw cannot be removed since it, and not the sides of the plaster, is holding the toggle in place.
Typically the size and weight of the item you are hanging will determine which anchor to use. In plaster walls, these expanding sleeves and molly bolts work well with light to medium weight items, up to about 30 lbs. Toggle wall anchors can usually handle more weight but they must be installed with the mounting hardware assembled to the anchor before installing the anchor into the wall.