Monticello is the Smalltalk most common source code versioning system. It was developed several years ago and implemented in several Smalltalk dialects. Monticello lets you commit a new version, update to a new one, merge, diff, revert, etc. Monticello there is no way to easily specify dependencies, identify stable versions, or group packages into meaningful units. Monticello just manages package versions. Metacello is another project, which manages package dependencies and the notion of projects.
There is no need to install Monticello since it is included in all Pharo distributions. For more information about Monticello, you can read the Pharo By Example Monticello chapter.