Green card lottery is performed by a computer. The computer green card lottery does not actually determine who will obtain the green card visa (because green card application is not exactly the same as green card visa).
The computer lottery was instituted to ensure fairness of application selection. As the green card application requirements are very basic, and there is only a limited amount of green card visas that are issued by american government every year, a fair method of selecting applications for final reviews is needed. The green card lottery is called to do just that.
As of recently, green card applications can only be submitted electronically via the Internet. Not only this helps prevent fraud and mistakes but makes handling of green card applications much easier. As there's no need to manually review all the applications, the computer lottery will simply draw a pre-defined number of them.
The applications drawn are considered winning and will be reallocated by zones, to be re-checked and to satisfy quotas. As soon as this is done, the winning applications will be processed and the winners will be notified and summonned.