Clear evidence that I am getting old and being left behind: of the four technologies named in that title I only have heard of one and have never actually used it.
All I got was that it's some sort of userspace microkernel geared around abstracting the computing resources of machines (nodes) in categories like CPU/RAM/disk and expose them programmatically through a cluster-wide API. The master daemon makes resource offers to a slave daemon, which in turn runs ported applications (be it Hadoop, Jenkins, ElasticSearch or whatever) called Mesos frameworks that have to be ported to this Scheduler/Executor paradigm (sort of like a cluster resource-level MapReduce, I presume?) in order to perform Tasks.
Mesosphere is the whole stack as an integrated OS. Marathon is pretty easy to understand - it's an init system/framework supervisor that operates on a higher level of abstraction than simple OS processes.
But yeah, it's a pretty tangled mess.
Mesos is a generic cluster manager. Its job is to keep track of available resources (RAM/CPU/Disk space) on a group of machines, and to share these resources between "Frameworks", which are more or less specialized distributed applications.
Available framework include existing applications that have been ported to Mesos (Jenkins, Hadoop, Elasticsearch, ...), along with some that have been developed specifically for Mesos (Chronos for running cron-like tasks, Marathon for running long-running tasks).
For example, you can tell Marathon "I want to run 10 instances of ./my_code, with 512MB of RAM and 2 cores each", and the framework and Mesos will collaborate to figure out where to run them. As a bonus, Mesos has some Docker integration, so you can natively do things like "Start 20 instances of container my_organization/my_app".
The datacenter OS thing is a bit of a shaky metaphor, but the idea is that, like the OS does for a single computer, the role of Mesos is to view your cluster resources as a single pool, and to try to fairly allocate these resources between multiple (distributed) applications.