Mundanely enough, it's just an overlay or overlap. The reason for it is twofold, and also somewhat mundane: 1) because we can (Photoshop makes it easy), and 2) it's the fashion.
This is one of those cases where technology drives fashion. Before Photoshop, this kind of effect required some very exacting work with a razor-knife on a big transparency. You'd see it done in high-end advertising work, but generally not on deadline-driven publications like magazines.
When it became possible to extract part of an image from its background easily and quickly, some creative designer realized it would make an arresting visual (to attract attention on magazine racks and newspaper stands) if the head of the featured cover-person overlapped the publication's nameplate. The idea caught on, and within a year or so everyone was doing it. It became a characteristic style that very few magazines (looking at you, National Geographic) haven't adopted.
As an aside, other recent examples of technology-derived fashion in graphic design are drop-shadows, which became ubiquitous because InDesign made them easy (and for which the former product manager of InDesign apologized to the design community at large!), and rounded corners, which came in with InDesign CS4 for print and are made easy on the web by CSS3.