Density Functional Theory (DFT) is a versatile and powerful quantum mechanical modeling technique extremely popular among physical chemists and material scientists, in particular. It employs a functional of spatial electron density to derive properties of many-electron systems. Since late 1960s, it has been commonly used for solid-state physics calculations, with good agreement with experimental data.
However, its use in quantum chemistry calculations has become more popular over the last 25 years due to advances in approximation methods that address exchange and correlation interactions. Open problems remain, principally in the treatment of intermolecular interactions (such as van der Waals interactions so important in organic electronics and biomolecular systems). This period of adoption has thus seen rapid changes in the field with much controversy and literature contributing to the advancement of understanding of the limits of applicability of DFT and improvements to it by many different theoreticians / scientists. With the commercial availability of software like Gaussian, the barrier to use of DFT by non-specialists (such as experimentalists) has narrowed significantly. However, like all other modeling techniques, DFT is not a black box, and its numerous subtleties can conspire to produce unphysical results that can lead researchers astray. For this purpose, it is important for all consumers of DFT-based methods to at least develop a basic understanding of DFT. This type of task has traditionally been challenging for many other modeling techniques as well.
To ease non-specialists (and young computational DFT experts) into the field, one of the more prominent quantum theorists, Prof. John Perdew (Tulane), has co-authored an article in the International Journal of Quantum Chemistry that provides fourteen easy lessons that will get you up and running with an understanding of non-relativistic DFT. This article is based on a seminal lecture given by Prof. Perdew at the Sanibel symposium. The lessons contain a paragraph or two (each) of condensed wisdom that often passes one by, in a more thorough reading of the numerous books and tutorials on DFT. This will probably not make you a DFT expert, but it will hopefully save you from knowing just about enough to be dangerous!