The canonical box is where a player is surrounded by blocks on all sides, but any directly targetted disruptive manœuvre can be used to construct a box. The difficulty in constructing a good box is of course in getting a player to stay still long enough to be surrounded. A well-timed shunt is often helpful here.
Ruttsborough once said "A loose box is no box at all," which illuminates one interesting detail of boxes: if a player is boxed into a large enough section of the map, then he might as well not be boxed in. Indeed it is not unknown for players who are on the run and need a breather to box themselves in as a defensive option (eg Tyburn v Ruttsborough 1963, where Tyburn restricted himself to a five station stretch of the Victoria line for fifteen turns while recovering from a typically blistering series of drivebacks. Not that it helped).
Note that boxing effects caused by collateral damage, especially randomly determined collateral damage, are not considered to be boxes per se since they are not the result of deliberately targetted action. Given that there is not usually any bonus given for boxing, this is rarely a significant point.