This is another one of those terms in DW / BI that gets really muddled. In fact, going through a few books, and a few sites, I didn’t find a single concise explanation. In fact, they are often intermingled. So, here’s my take, in simple words.
Data Governance: These are the guys who write the laws. They define the rules and regulations that govern your data. They are Congress.
Data Stewardship: The enforcement, and maintenance, of your data rules and policies. These are your cops, or to confuse the issue, because this is BI and the language has to make the easy, confusing, data governors.
Now, this is where it gets confusing. Data stewards often are also part of data governance. In other words, it’s like a cop being elected to Congress but he keeps his day job as a cop. They do both.
Still, and I think for a simple explanation, Congress and cops makes for a respectable explanation. So, who are the robbers? The chuckleheads entering data into your systems.