A reciprocating saw is basically any saw with a blade that cuts with a back and forth motion, such as a jigsaw. Some reciprocating saw blades, such as the jigsaw, have a limit to the thickness they can effectively cut. A jigsaw only works at about an inch and a half, any thicker material and you can get inaccurate cuts and an overheated blade. Never force such a blade - it will eventually snap. Reciprocating blades are more prone to breakage than other types of blades, it's best to have a good set of spares on hand when doing an extended project.