The easiest way how to switch AC with arduino is use of digital PINs and triac. Digital PINs can output 5V up to 40mA which is enough to turn on optotriac. And then it depends on maximum current the triac can handle.
Schema from MOC3041 datasheet:
Digital output PIN of arduino can supply DC up to 40m which is not enough for most application we need (except of switching few LEDs:)). The solution is in use of transistor. Alternatively one can use the combination on transistor and electromagnetic relay. By this combination we even get electric isolation between arduino and switching part of circuit.