A cherrypick is an operation in which the delta between two revisions is applied to a working tree. The process is roughly similiar to generating a diff between two revisions and applying it to a working tree.
A cherrypick is a merge in which a range of revisions are selected and merged into a working tree . Performing a limited merge is different than a normal merge because normal merges imply all missing revisions should be merged. One of the results of cherrypicking is the creation of intransitive ancestors, which are revisons that are missing one or more of their ancestors.
A cherrypick in Bazaar is a bit more advanced than the cherrypick with some other revision control systems, in that a three way merge is performed rather than a strict delta between revisions.
The downside to cherrypicking is that Bazaar does not track cherrypicked revisions, although this feature is firmly on the wish list.
A common use-case for cherrypicking is the back-porting of fixes from an ongoing branch of development to stable release branches. For a discussion of this, and how to avoid it, see http://monotone.ca/wiki/DaggyFixes/