A clause is defined as...
...the smallest grammatical unit that can express a complete [meaning].
There are five different kinds of clauses:
declaratives, open interrogatives, closed interrogatives, exclamatives, and imperatives.
Each of these is generally connected to a "specific speech act." For example, the interrogative is used in order to ask a question. It is also the interrogative clause that is referred to as open or closed. When these distinctions are made, the "intended speech act" makes a "command"—a request for information.
A closed interrogative basically allows for a limited number of responses: "yes" or "no." (They can be called "yes/no questions.") However, with an open interrogative, the range of possible responses is much greater.
If one asks, "Do you want a slice of cake?" (a closed interrogative), the responses will be an affirmative or a negative. However, if the question is, "Where do you want to go for dinner" (an open interrogative), the number of responses could go on and on.
In addition, "closed interrogatives can be divided into two types: polar and alternative." The polar question will be answered by a "yes" or "no" response (as in "polar opposites"), where an alternative offers just that—an alternative, or a choice.
For example, "Do you want a slice of cake?" is polar: the answer will be "yes" or "no." However, for "Do you want a slice of cake or a piece of pie?", the answer will be the choice between the two alternatives, making the question an alternative closed interrogative.
On the other hand, open interrogatives are "information-seeking" questions, and make use of "wh- words." These kinds of words begin with "wh-" and include:
who, whom, whose, what, which, when, where, why, how
So open interrogatives will look like:
Who is going to the movies with you?
What toppings do you want on your pizza?
For each of these examples, note that they are open because they require more than a "yes" or "no" answer, and the range of responses can be extensive.