When a teacher asks the class a question and then calls on someone to answer, everyone else breathes a sigh of relief. Many completely tune out the next minute or so feeling that they are "off the hook" for now. Often, one student blurts out an answer before others can even think of an answer (providing they want to participate). This typical scenario is not conducive to learning. There are better ways to ask questions.
One option is to use "wait time." This is taught in teacher education courses as one of the best question-asking strategies. But, this simple concept is not so simple to implement. The idea with wait time is not to immediately call on someone to answer the question, but rather give several seconds of think time before calling on someone. Theoretically, every student is coming up with their response and are thus involved in learning. When implemented well, students do well with wait time, and it does increase learning. However, when one or two eager, impulsive students answer even when asked to wait, the strategy fails.
A better option is to use "writing to learn." Every student is asked to write their response to your questions on a scrap sheet of paper. This takes about the same amount of time as wait time for most questions and assures everyone is involved. It also satisfies the need for impulsive students to answer immediately. When everyone finishes writing their response, the teacher calls on one or two, gives immediate feedback and moves forward. These writing to learn activities are never graded so students are willing to participate wtihout fear. Writing to learn is appropriate at the beginning of class to review previous work, all throughout the lesson to ensure involvement, and at the end of class to make sure everyone learned today's lesson. It is important to vary the type of questions and to make sure they do provoke thought rather than only ask for recall of information.
So, teachers--make sure all your students are involved in class and the questions you ask are learning experiences for all your students.