Yeast is what makes wort into beer. It's a microorganism that likes eating sugar - when it consumes sugar, it emits carbon dioxide and alcohol. Give it food, and it will reproduce sufficiently to brew beer. Once you've brewed a bit you'll be able to gather the used yeast once your brew's first fermentation is complete and gather it up to store in a fridge where it will remain dormant, but at beginner level, it's easier to buy it preprepared in a packet in a homebrew shop. It's quite cheap (1.50 per packet or so) so that's an easy option.

The yeast you use influences the flavour of your beer very subtly - different yeasts with the same malt and hops will produce very subtly different beers.