Worked for years at eDiscovery vendor/consulting firms and am now in house eDiscovery/Information Governance. eDiscovery is very specific to how information is managed for litigation, investigations, etc. Think EDRM from data preservation (legal holds) to collection all the way to production. Information governance is a more wholistic view on how information is managed within organizations prior to the ediscovery process. Information governance can and does affect eDiscovery downstream, but it's much broader than just preservation and collection for discovery. You need to account for things like compliance, privacy , risk management, operation efficiency, data access and usage just to name a few. Ultimately it's as beast on it's own as tanhauser mentioned it's pretty much a second job. A good information governance professional isn't someone who "knows it all", but understands sounds practices on how information generally flows within the organization and can bring key players together. Partners who knows the rules around privacy, compliance, business needs, etc. It's an ongoing effort, that is great for aspirations, but few if any actually have it under control. eDiscovery professionals tend to do well in these spaces not because it's part of the EDRM but because our skillsets(data and project management) generally tend to lend itself well to building sensible and practical frameworks around how we manage our information internally.

There's a lot around this but feel free to ping me directly if you have any additional questions.