Help: Region code
A Region code is a technical feature on DVD and BluRay discs that seeks to prevent customers from a certain area of the world to play and watch discs produced for another. Simply put, a disc of one region does not normally play on devices produced for another.
DVD
DVDs have 8 possible regions, 0 being equal to "region free" (i.e. not region-coded), from 1 (Northamerica), 2 (Europe) to 6 (mainland China). Learn more on Wikipedia.
Circumvention and technical implementation: DVD region code is software-based on players, that means a change in the software of a player can typically render it multi-region. Other options are technical modifications. It is generally fairly easy to find players that either are region free or can be made multi-region.
BluRay
While there are less regions for BluRay. A (Northamerica, Japan, etc.), B (Europe, Africa, Australia, etc.) and C (Asia). Learn more on Wikipedia, discs/devices are additionally country-coded which means that a region-A disc from Japan may not necessarily play on a US-bought player device.
Circumvention and technical implementation: BluRay region code is hardware-based, that means in order to change the region coding or to disable it, (typically expensive) hardware modification is required. It is therefore harder to obtain BluRay players that are multi-region (vs DVD-players), in some cases it may even be cheaper to procure two or three sets of players instead. Some multi region players sold are also only region free on their DVD capability.