Isis is a simple, yet powerful database system with a large installed base since the 80s. Since it's well suited for bibliographic data, it's commonly used in libraries, and since it's very low cost, especially in those running on a low budget.
Introduction to the isis db
An isis DB is a list of rows of unspecified structure, each identified by a unique number, the rowid (a.k.a. mfn).
Each row is a list of fields, and each field has number (tag) and a string value. Within a row there may be zero, one or more fields with a given tag. While the field's value usually is a textual representation of data in one or the other character encoding (commonly one of the IBM/DOS code pages), it may actually contain arbitrary bytes. (Extract from http://www.openisis.org )