Plastic explosives are, pound for pound, more powerful than dynamite, causing 40-80 points of damage, and sports a wider blast radius. Correspondingly, plastic explosives are more expensive than dynamite.
Plastic explosives have a timer that can be set to anywhere between 10 seconds and 3 minutes, in 10-second increments.
In Fallout, plastic explosives must be dropped manually, even if held in the active item slots. In Fallout 2 however, if they are activated in the active item slots, they drop automatically.
Plastic explosives are harder to come by when scrounging, but military facilities often have at least a few charges in stock. Most shops do not carry plastic explosives, even if they carry dynamite. In terms of shopping, the best bet is to try higher-end weapons shops. Along with their big-ticket guns, some may have a few bundles of plastique.