Too Long; Didn't Read
Sharding is a hub-and-spoke architecture with a central "main chain" that serves as the consensus backbone of the system. Plasma chains are sidechains that have a non-custodial property: if there is any error in the Plasma chain, then the error can be detected, and users can safely exit. Sharded chains with no hub are theoretically possible but haven't been done so far; this article will not focus on them. Plasma is not a tightly-coupled system: an invalid Plasma block absolutely can have its header be committed into the main.