Enhanced sampling algorithms have emerged as powerful methods to extend the potential of molecular dynamics simulations and allow the sampling of larger portions of the configuration space of complex systems. This review aims to present the unifying principles and differences of several computational methods for enhanced sampling in molecular simulations of biomolecules, soft matter and molecular crystals. Indeed, despite the apparent abundance and divergence of such methods, the principles at their core can be boiled down to a few statistical and physical principles. To enable comparisons, the various methods are introduced using the same terminology and notation. We then illustrate in which ways many different methods combine principles from a smaller class of enhanced sampling concepts. This review is intended for scientists with an understanding of the basics of molecular dynamics simulations and statistical physics who want a deeper understanding of the ideas that underlie various enhanced sampling methods and the relationships between them. We expect this to help scientists make informed decisions about which method to use. This "living" review is intended to be updated to continue to reflect the wealth of sampling methods as they emerge in the literature.