This has been discussed in various places, and people have different opinions. But so far, my opinion is that a POD is (or can be) a Data Wallet, but there’s nothing specific to the idea of a “Data Wallet” that can’t be achieved with a POD.
In particular, a “Solid POD” is a server that supports the Solid Protocol. But as far as I know, there isn’t specification or anything else defining what a Data Wallet actually is. So in my opinion, it’s just a buzz word or a marketing term to make “PODs” more appealing. The only technical description I’ve heard is that a wallet is a place that stores signed documents (with things like Verifiable Credentials and such). But you can do that in a POD, and use client-client specs (like Type Indexes) to coordinate among clients.
Then, there is Inrupt’s Data Wallet, but in my opinion that’s not even Solid as I discussed in this thread: Inrupt's Data Wallet.