For purposes of storage, there are basically two types of seed: 'desiccation-tolerant' and 'desiccation-intolerant'. Most of the garden plants with which we are familiar produce desiccation-tolerant seeds, which means they can be safely dried for long-term storage. Exceptions include many aquatic plants, large-seeded plants, and some trees (such as oaks and buckeyes), many of which produce desiccation-intolerant seeds and will die if allowed to dry.
Desiccation-intolerant seeds do not enter dormancy after maturing. Instead, respiration and other physiological processes continue. Continued respiration after maturation causes desiccation-intolerant seeds to deteriorate rapidly once they have matured, so they must be planted while still fresh. Desiccation-intolerant seeds partially or completely lose viability if they are allowed to dry—usually they die.
Some seeds (such as citrus) are 'borderline' desiccation-intolerant. These can be dried and stored for some time, but lose viability quickly and germinate slowly once they've been dried.