Not quite. It’s a subtle difference, but Execution Environments are not the same as shards. A single shard may have more than one EE, and an EE can exist in multiple shards. The concept of EEs is an abstraction of the EVM; basically, the Eth2 system will be able to handle more than one virtual machine. The most-used EE will likely be the EVM (eWASM), but in my understanding other EEs could exist that model state execution in different ways, e.g. a UTXO-based system, or Libra, or lots else.
There isn’t actually a great single resource that explains EEs (at least that I’ve been able to find), likely because its still an evolving concept. My current understanding has come from cobbling together bits and pieces from various places (and is probably incorrect to some degree!). If anybody more knowledgeable than I about EEs wants to create such a resource, I’m sure the community would greatly appreciate it .