Typically, a security that is exempt from registration is restricted in some way. See the first post on Restricted Securities, but keep in mind that some exemptions provide for other restrictions.
In finance, a security is a negotiable financial instrument that holds some type of monetary value. A security can represent a number of positions. Typically, securities represent ownership in a publicly-traded corporation, known as equity securities, or represent a creditor-debtor relationship, known as debt securities.