There are two types of limiting: cell and face. Cell limiting determines the limited gradient along a line connecting adjacent cell centers. Face limiting determines the limited gradient on the face itself. Cell limiting should be less dissipative.
Standard gradient limiting, i.e. cellLimited, clips each component of the gradient equally (remember it's a vector). cellMDLimited is a Multi-Dimensional limiter whereby the gradient is clipped in the direction normal the cell faces.
Of course, reducing dissipation has the potential benefit of better accuracy but increases the risk of instability. The limited gradient schemes in OpenFOAM listed from least to most dissipation:
If you wanna look yourself, the source code for the gradient schemes can be found in the following directory:
This article is based on a forum post by Chris Sideroff: http://www.cfd-online.com/Forums/openfoam-solving/81486-cellmdlimited-vs-celllimited.html