Complex models in physics, biology, economics, and engineering are often ill-determined or sloppy: their multiple parameters can vary over wide ranges without significant changes in their predictions. This review uses the tools of information geometry to explore this phenomenon, and the deep relations between sloppiness and emergent theories. We introduce information geometry, the model manifold of predictions whose coordinates are the model parameters, and its hyperribbon structure. These hyperribbons explain why only a few parameter combinations have significant effects on the behavior of complex multiparameter models. We review recent rigorous results using approximation theory to connect the hierarchy of hyperribbon widths to approximation theory, and to the smoothness of model predictions under changes of the system control variables. We discuss recent methods for following sloppy geodesics to find simpler models on the boundary of the model manifold -- emergent theories with fewer parameters that explain the behavior equally well. We discuss a recent `optimal' Bayesian prior which maximizes the mutual information between model parameters and experimental data. This prior naturally favors points on the emergent boundary theories, thus selecting simpler models when they make nearly equivalent predictions. We introduce a `projected maximum likelihood' prior that efficiently approximates this optimal prior, and contrast both to the poor behavior of the traditional Jeffrey's prior. We discuss the way the renormalization group coarse-graining in statistical mechanics introduces a flow of the model manifold, and connect stiff and sloppy directions along the model manifold with relevant and irrelevant eigendirections of the renormalization group. Finally, we discuss recently developed `intensive' embedding methods, allowing one to visualize the predictions of arbitrary probabilistic models as low-dimensional projections of an isometric embedding, and illustrate our method by generating the model manifold of the Ising model.