I find Getzler and Kapranov's article <a href="http://arxiv.org/abs/dg-ga/9408003">Modular Operads</a> difficult to understand. Can anyone explain what a (twisted) modular operad is conceptually, or what the underlying idea behind the concept of a modular operad is? For example, how can we write down the explicit structure map in a twisted modular operad?