Buildbucket is a generic build queue. A build requester can schedule a build and wait for a result. A building system, such as Buildbot, can lease it, build it and report a result back.
Buildbucket has native integration with Swarming and Recipes. A bucket can define builders and a buildbucket build in such bucket is converted to a swarming task that runs a recipe. The results are reported back to buildbucket when the task completes. See Swarming.
Buildbucket is integrated with buildbot. You can schedule, cancel, search for buildbot builds and check their results using buildbucket API.