There are two main types of NDA: mutual or one way (sometimes called unilateral). The "Who is protected?" position allows you to select which of these applies.
A one way agreement places someone under an obligation to keep your information confidential. But you do not agree to keep their information confidential. This is appropriate where only one party will be disclosing information (e.g. you provide a set of confidential information to a service provider to prepare a report). In a one way NDA, you naturally need to be clear about which party is taking on the obligations.
The other option is a mutual NDA where the obligations flow both ways (e.g. each party may disclose information to the other party). This is often used where two parties are negotiating a business deal and in the course of those negotiations confidential information may be disclosed by each party at various times.
A well-worn negotiating path for an NDA goes something like this. The first party proposes a one-way NDA. This protects their information but doesn’t expose them to any liability. However, the second party reviews this document and suggests it is not really appropriate. The second party wishes to disclose confidential information of its own as part of a discussion. They want their own information to be protected in the same way. The parties eventually agree a mutual NDA.