For the purposes of the Buildbot, we will try to generalize all VC systems as having repositories that each provide sources for a variety of projects. Each project is defined as a directory tree with source files. The individual files may each have revisions, but we ignore that and treat the project as a whole as having a set of revisions (CVS is the only VC system still in widespread use that has per-file revisions, as everything modern has moved to atomic tree-wide changesets). Each time someone commits a change to the project, a new revision becomes available. These revisions can be described by a tuple with two items: the first is a branch tag, and the second is some kind of revision stamp or timestamp. Complex projects may have multiple branch tags, but there is always a default branch. The timestamp may be an actual timestamp (such as the -D option to CVS), or it may be a monotonically-increasing transaction number (such as the change number used by SVN and P4, or the revision number used by Bazaar, or a labeled tag used in CVS)1. The SHA1 revision ID used by Mercurial, Git and Monotone is also a kind of revision stamp, in that it specifies a unique copy of the source tree, as does a Darcs “context” file.
When we aren't intending to make any changes to the sources we check out (at least not any that need to be committed back upstream), there are two basic ways to use a VC system:
Build personnel or CM staff typically use the first approach: the build that results is (ideally) completely specified by the two parameters given to the VC system: repository and revision tag. This gives QA and end-users something concrete to point at when reporting bugs. Release engineers are also reportedly fond of shipping code that can be traced back to a concise revision tag of some sort.
Developers are more likely to use the second approach: each morning the developer does an update to pull in the changes committed by the team over the last day. These builds are not easy to fully specify: it depends upon exactly when you did a checkout, and upon what local changes the developer has in their tree. Developers do not normally tag each build they produce, because there is usually significant overhead involved in creating these tags. Recreating the trees used by one of these builds can be a challenge. Some VC systems may provide implicit tags (like a revision number), while others may allow the use of timestamps to mean “the state of the tree at time X” as opposed to a tree-state that has been explicitly marked.
The Buildbot is designed to help developers, so it usually works in terms of the latest sources as opposed to specific tagged revisions. However, it would really prefer to build from reproducible source trees, so implicit revisions are used whenever possible.
 many VC systems provide more complexity than this: in particular the local views that P4 and ClearCase can assemble out of various source directories are more complex than we're prepared to take advantage of here