  In this survey article we introduce the notion of frontals, which provides a
class of generalised submanifolds with singularities but with well-defined
tangent spaces. We present a review of basic theory and known studies on
frontals in several geometric problems from singularity theory viewpoints. In
particular, in this paper, we try to give some of detailed proofs and related
ideas, which were omitted in the original papers, to the basic and important
results related to frontals.
