• Letting them ask questions--where an audience can participate in a q&a session
• Rhetorical questions--asking an audience questions that aren't intended to be answered
• Questions to shape the presentation--where the answers to the questions you ask will you actually change your presentation
• Knowledge questions--gauging what your audience knows about a topic
• Competition questions--reviewing what you've told your audience to ensure comprehension
• Questions to get demographics--finding out where your audience is at on a particular subject/topic.

Rhetorical Questions: All questions have their place, of course, and they're a fantastic way to engage the audience. We naturally answer a question that's been asked within our heads--whether we do it consciously or not. That's why good speakers (I'm talking super-top-tier) will often pepper their speech with cues that get audience buy-in/engagement. Sometimes that's as simple as saying "right?" at the end of a point--at which time the audience will generally nod or make a physiological sign of agreement/engagement. It's not intended to be answered, and it's only effective if you know how to use the strategy. Too many rhetorical questions become a handicap to a good speech instead of an aid.

Q&A: Letting the audience ask questions: is a structured-unstructured process. I often find it more successful to let the audience write down questions and then have a specific, separate time to address them. This isn't to prevent the "flow" from being interrupted, but is, rather, another strategy for engagement. Lots of audience questions are very specific to a particular situation (that no one else in the audience might have) or are irrelevant to the topic. Or people ask questions seeking prominence and recognition with the speaker or their peer group. They can be really dull if not managed. (Not all of them--there are times when a good audience question session ends up really making the presentation great, but from what I've seen this is a rarity.)

Demographic and Shaping Questions: Asking the audience questions that are meant to be answered can certainly be a way of engaging them. Either you review knowledge by asking them rapid-fire questions about what you just covered, or you use the questions to gain datapoints/craft your presentation. The latter, of course, is only useful if you're willing to change things on the fly. There's nothing worse than being asked one's opinion only to have it thoroughly ignored--what's the point in that?

So if you're asking your audience how many of them have children to prove a point--and no one does have children--you should be prepared to shift your point instead of plowing forward. If the presentation needs to be tailored in reaction to a demographic question, be prepared to do so.

Knowledge and Competition Questions: Asking the audience questions as a review--either in competition context or just to self-test their knowledge--can be one of the most engaging tools in a speaker's toolbox. HOWEVER, this needs context within a presentation. One must prepare the audience to answer questions if they're not going to be in a structured game or competition. Prime the audience and give them permission to speak up--and let them know how to do so. If you have a multiple choice question on a powerpoint, are they supposed to shout out an answer? Vote? Raise their hands? Is it rhetorical or not? Clarifying this for your audience is key.

Competition questions can boost content retention--and presentation attention--exponentially. When the audience knows that they're going to be competing with their peers, they hyper-focus on the information at hand to gain the advantage and contribute to their team.