Most commonly, NDAs are "mutual" - both parties agree not to disclose the information exchanged. A mutual agreement makes great sense since in almost every situation the two parties have a mutual interest in constraining confidential information.
In some cases, however, you might be asked to sign a one-way NDA. A one-way NDA contains only one party from disclosing information exchanged. Party A may disclose information to Party B who must promise to Party A not to disclose this information. Usually, one-way NDAs accompany a piece of information that is transfered non-verbally. Party A might send specification or a product plan to Party B - and Party B discloses no information to Party A.
There is a logical argument that one-way NDAs offer no real benefit over MNDAs since, even in the case of a one-way transmission of information from Party A to Party B, Party A's inclusion in an agreement not to disclose information from Party B (where no information is actually provided) are the "sleeves off the vest" of Party A yet provide a simple layer of protection to Party B should there be a subsequent communication exchange over the information. Starting with and MNDA covers the one-way case while also saving time and trouble by inherently covering future information exchanges in both directions.