Riot could be considered the flagship Matrix client. It is made by many of the same people that write the Matrix spec but it is just one Matrix client. Your account works with any Matrix client. You can find many more using Try Matrix Now
Riot web is...
Riot Android is a native Matrix client for Android.
RiotX is the code name for the future version of :term:`Riot Android`. It is a rewrite of Riot Android written in Kotlin and a new radically different UI.
RiotX is just the future version of Riot Android. It is a complete rewrite to make it more performant and to completely change the interface. RiotX is just a name to seperate it from Riot Android while it is being developed. It will become Riot Android in the future.
Riot iOS is a native Matrix client for iOS.