If you're new to sound systems, whether your shiny new speaker set-up needs a receiver may crop up quite early. Older surround sound systems always needed a receiver for the best performance, and while they're still an option, many systems now operate without needing one.
Soundbar systems (those with a bar, subwoofer, and 2+ rear speakers), which are becoming more popular, won't require a receiver at all.
However, multi-speaker systems will depend on how traditional you want to get. A/V receivers are the glue that holds the rest of the components together, and so one may be needed for home theater systems. Before investing in one of these set-ups, ensure you have everything you need to start.