A vector format, such as postscript or svg!
Vector formats do not use pixels and therefore can be scaled and zoomed without ever becoming pixelated. Note that when the logo is actually used (e.g., on a web-site), it will typically be rendered as a bitmap (e.g., PNG image) of a certain size (number of pixels).
Mark is gonna be the answer for question 1
Sorry for this but to be honest we do t get taught this in the school I go to sorry