The most critical unit in a database is the record. If you think about it, most databases are arranged around a certain kind of entity. A phone book is a collection of telephone companies. A card catalog is an index to a collection of books in a library. You might also have a database referring to diseases and viruses, giraffes in a zoo, video tapes in your library, or anything else. The important idea here is that you can think of a database as a collection of like things. These things don't even have to be real. You could just as easily have a database of dreams, monsters, or concepts. What really matters is that all the things in the database have to be similar. For example, you might have a database of fruit in a supermarket. You might have entries in this database describing bananas, pears, oranges, and apples, but an entry describing a station wagon might not fit here. It also may not make sense to have an entry for pork chops in this database, even though pork chops might be in the same supermarket.
Each entity in the database is one record. If your database describes a bunch of giraffes, one record in the database describes one giraffe. If your database describes fruit, one record might describe one type of fruit.