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ISAM stands for Indexed Sequential Access Method. ISAM was originally developed by IBM and antedates the concept of the relational database. ISAM stores data sequentially in a file. This data is typically records for a database, though the term has grown beyond its original database-specific meaning.
The records are composed of fixed length fields. This allows for sequential reading and writing of records to the database. In order to speed access to the records, ISAM adds an index to the file which allows for random access of any record simply by looking up some key field of the record in the index, just as a human might look up a page number in a book by looking for a word in the index at the back of the book.
This sort of database methodology is often referred to as a "file-based" system. This older technology is very simple to understand and implement, as it primarily consists of direct, sequential access of the database file. It is also very inexpensive. The tradeoff is that each client machine must manage its own connection to each file it accesses. This, in turn, leads to the possibility of conflicting inserts into those files, leading to an inconsistent database state.
ISAM was replaced at IBM with a methodology called VSAM (Virtual Storage Access Method). Still later, IBM developed DB2, which, as of 2004, IBM promotes as their primary database management system. DB2 is a relational system, while ISAM and VSAM antedate the concept of the relational database.
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