A bilateral, or mutual, agreement is where both parties will be supplying information that is intended to remain secret. This type of agreement is common when businesses are considering some kind of joint venture or merger.
Some practitioners insist on a mutual NDA in all cases, to encourage the drafter to make the provisions "fair and balanced" in case the drafter's receiving-party client later ends up as a disclosing party, or vice versa (not an uncommon occurrence)[