Order imbalances happen on stock exchanges such as the New York Stock Exchange when there are too many buy orders and not enough sell orders -- or vice versa. When they occur, the exchange might halt trading temporarily, to allow more of the other kind of order to come in. This permits better matching of buyers and sellers, and it makes prices less volatile.
You might see order imbalances happen whenever there's very good or bad news related to a company, and suddenly many people want in or out of it.
The Nasdaq operates on more of a supply-and-demand basis, with its trading conducted among many market participants. It doesn't halt trading for order imbalances.
To learn more about investing Foolishly and how the business world works, visit our Fool's School and our Investing Basics area. Or check out some of our inexpensive and well-regarded online how-to guides, which come with money-back guarantees.
You can also learn all about brokerages and find one that's right for you by visiting our Broker Center. (Did you know that some well-regarded brokerages are offering commissions as low as $5?)