Monorepos (Monolithic Repositories) are used by large companies, such as Google and Facebook, and by popular open-source projects, such as Babel and Ember. This study provides an overview on the definition and characteristics of monorepos as well as on their benefits and challenges. Thereupon, we conducted a multivocal literature review on mostly grey literature. Our findings are fourfold. First, monorepos are single repositories that contain multiple projects, related or unrelated, sharing the same dependencies. Second, centralization and standardization are some key characteristics. Third, the main benefits include simplified dependencies, coordination of cross-project changes, and easy refactoring. Fourth, code health, codebase complexity, and tooling investments for both development and execution are considered the main challenges.