PageRank sculpting is the use of nofollow links to channel the PageRank from the source page to targeted outgoing links, at the exclusion of the nofollow links on the page.
Google changed their approach to PageRank to ensure that rank still is allocated or 'lost' to the nofollow pages. Consequently, the target pages do not get a disproportionate allocation of the outgoing PageRank.
While PageRank sculpting is not encouraged by Google, there is recognition that a certain amount of sculpting is necessary; particularly for pages that are not important to the flow of information (e.g. a checkout link for a shopping site).