This type of relationship presupposes connection of a record stored in the Master table with many records of the Detail table. For example, in a Market Research firm each client can order many projects (researches), while every project can be ordered by only one client. As a result, each record of the Client master table can be related to many records of the Project detail table.
The relation of the one-to-many type is set between a key column in the master table (using a key column ensures the uniqueness of a record to be related) and a column of matching type in the detail table.
Please note that One-to-Many relation is realized at the level of columns: a column in a master table is related with a column of matching type in a detail table.
Next: Many-to-Many Relations