Hi Daniel,
You have asked a good question on PMBOK 8, page 274 it gives the definition, I invite to read it and there are other examples in the guide. However there is a very important part of the definition, which is a steering committee can make decisions outside the team’s authority. Its also a broad governance body, That’s important to know in the exam as you may be asked questions on who can make a decision and its important to know where the Project team end and the steering begin.
It doesn’t deal with daily tasks but decisions above the PM authority such as approving major scope changes, approving phase gate approvals. Now its important to know that the terms Steering committee, may not always be used but it’s important to know its people who can make decisions above the PM, for example management reserves, which “could be” be under such control. I recommend going through these terms in the PMBOK guides and you quickly get to terms with it. Just remember its a broad body for the entire project dealing with major desicion of the projects once above the single remit of the PM.
