From Unofficial BOINC Wiki
In a Relational Database a unique value that completely identifies a row within a database table. This unique value may be a simple integer which is incremented, usually by one, for each new record added to the table. The Primary Key can also be a set of data values within the record that when used in conjunction with each other will uniquely identify a record (usually called a concatenated key).
For example, my name, "Paul D. Buck" does not necessarily uniquely identify me, even though most people feel that one Paul D. Buck is more than enough for anyone. Even when used with my address, this is not enough information to guarantee a unique identification of a particular person (think of a school for example, same address, but there may be one or two people there with the same name).
This is the reason that database designers tend to use a generated unique key.