Packages are the units of versioning used by Monticello; the classes and methods they contain are recorded and versioned together. Monticello uses the packages defined by PackageInfo.
A Snapshot is the state of a Package at a particular point in time
A Version is a Snapshot of a Package and it's associated metadata (author initials, the date and time the snapshot was taken), and the Version's ancestry (the list of Versions from which it is derived). A Version is the standard currency of the system. You save them, load them, give them to others, merge them, delete... you get the picture. Versions are often stored in mcz files - see File Format
Each package in an image that is being versioned with Monticello has a Working Copy. The Working Copy represents the Version of the package that is currently active in the image, and which may be modified by the Smalltalk development tools.
These are places to store your Versions. Unlike CVS, in which a Package is associated with one Repository, a Monticello Package can have Versions in many repositories. When adding a new Repository to use, you can choose from SqueakMap Cache, FTP, HTTP (webdav), SqueakMap Release, SMTP, or a directory somewhere on your hard drive (or network drive). For example, if I have six versions of package Foo, I could have Foo versions 1-4 being on my local harddrive, and 5-6 being on an ftp server. You could download version 5, make some changes and commit a new version (7) to your WebDAV repository. I can download and merge that version with my own work to produce version 8, which I save to my ftp repository. This is a key element of Monticello's distributed development model.
The package-cache is a local repository the Monticello uses to cache any package that is loaded into a particular image in a directory. That means it is filled with .mcz files, whether it is a package you create in your image, or one you download from somewhere else. When you use images in different directories you will have multiple package-caches, and may hold many of the same packages. If MC is loaded into an image which is subsequently moved, MC will continue to use the package-cache in the directory the image was moved from. Otherwise MC creates a new package- cache in the local directory. This can become a real mess and so some have used symlinks on unix systems to centralize it.
When a Version is loaded into the image, it is likely to become the ancestor of new versions that are created as part of the development process. During merges, Monticello needs to examine the Snapshots of these ancestors in order to detect conflicts. By caching these ancestors as it loads them, MC reduces the chance that the necessary version will be unavailable - either because the repository it's in is no longer available or because it was loaded directly from a file and isn't in any repository.