It’s quite simple, arduino can control motors and receive information from sensors, but arduino input and output are limited in current and voltage. (I think 40mAmp and 5V)
It’s means that if you need to control a motor which need more power, (for example a 6V-200mAp DC motor) then you will have to add some electronics components and an external power supply.
And if you want to receive informations from a sensor which give an analog tension between 0-10V then you will have to put few components to protect you board and adapt the signal.
It all depends of what you want to do, but it’s quite easy even if you dont know the basic of electronics.
So finally if you just want to discover arduino and all that stuff around, you should buy a starter kit like this one http://www.sparkfun.com/products/9905 which come with some sensors adapted to the arduino and some basic components (like resistor) you will certainly need.
And then if you want to connect something else you sould first have a look to the “playground” section of the arduino website where they explain how to connect all sort of things to arduino (like step motor)http://arduino.cc/playground/
And for the next time you should ask your question to the arduino forum which is full of nice arduino experts http://arduino.cc/forum/, this forum is more specialised in vvvv software, so if you have a problem to connect arduino to vvvv then you can ask your question on this forum.
Look at this too io