# What exactly are the ADE type of gauge theories?

What exactly are the ADE type of (susy) gauge theories? What exactly we mean, intuitively, the ADE singularities? What are their relation to brane constructions and do you have any references one could understand the basics?

I tried to read a little bit of today's paper by Vafa et al. but could not even get the introduction. I would appreciate any information.

2. refers to finite subgroups $\Gamma$ of $SU(2)$, which is related to orbifolds $M/\Gamma$, i.e. manifolds with singularities. See also elementary catastrophes.