- everyone queues up in one line, and goes to the next open service window/register/person (often seen in banks these days, and at airport check-in counters); or
- people join a queue behind one service window or another. Typically, people will join the shortest queue. (This is what you typically see in fast-food outlets and supermarkets.)
The down-side, is that the single queue *looks* longer, and can increase the potential for people baulking. But, the queue also moves much faster, so people are also less likely to abandon the queue altogether.
When designing your service, and looking at your layout options at check-out, it's important to keep these things in mind.