Welcome to the the InterSystems Developer Community!!
1) ObjectScript is a (very, very large) superset of the M language (previously known as MUMPS)
2) InterSystems IRIS uses much of the core technology that was in Caché, but it is a break from the past in many ways and allows next-gen development tools, architectures, etc. InterSystems IRIS is an upgrade path which current Caché based applications should seriously consider. Zen is a legacy web development technology which runs on top of Caché (it ships with Caché and InterSystems IRIS)