Open Database Connectivity. A collection of related data, i.e. music titles, bank transactions, phone calls. Files, records, fields. SQL is a high level interface written specifically for databases. Use a locking methodology to prevent more than 1 user from changing data.
Most db languages have a way to lock , files, records, and some even fields.
Basically it is a table. SOme databases can have duplicate keys, but this means that a specific query could bring up multiple records which is not always desireable. For speed of searching and ease of managing the data. Because the index file is much smaller, and because algorhythms can be used to search thru the sorted index file faster. Microsoft Access is a good tool for managing and creating databases.