It's literally anything that doesn't fit the narrow description in the business visa. A random example is if you own a house in the US, and you do any maintenance work on your own house, that is work.
From the US web site
participating in business activities of a commercial or professional nature in the United States, including, but not limited to:
Consulting with business associates
Traveling for a scientific, educational, professional or business convention, or a conference on specific dates
Settling an estate
Negotiating a contract
Participating in short-term training
Transiting through the United States: certain persons may transit the United States with a B-1 visa
Deadheading: certain air crewmen may enter the United States as deadhead crew with a B-1 visa
Interesting. I used to work for a big international corporation and visited the US twice for work. A six week customer support gig and a one week field testing trip. The first trip was still under older visa waiver rules for Europeans, the second one under ESTA.
In none of those cases there weren't any kind of tricky questions asked at the border, I am sure I would have given the wrong answer. But I have been to more aggressive inquiries on conference trips, when giving the right answers was easy.
I have always assumed working means having a contract with an US employer (or of course self-employed).
One would assume a large corporation had processes in place not to expose their employees to such personal risks.
It really depends on how much such travel happens in said corporation. I've worked for companies that handled this well and others where they were generally clueless. Most large companies will have immigration lawyers that work with them but usually the trivial business trip doesn't go through a lawyer.
How much and what types of questions you're asked can depend on many factors. Whether the officer had a fight with his wife, your profile, how his day has been going etc. Sometimes it's "have a nice day" and sometimes it's a longer interrogation including secondary inspections.
This is a common misconception that work means having a contract with a US employer. As another example you're not allowed to work remotely for a non-US employer while physically being in the US.
From the US web site
participating in business activities of a commercial or professional nature in the United States, including, but not limited to: