filter is to screw on over the lens. depends on what lens diameter it is (eg 52mm, 67mm, 77mm). the filter is to prevent finger/water/rain splash /dirt/dust/whatever rubbish etc from coming into direct contact with the lens glass front element surface. the other function is since its a UV filter, it will supposedly filter out UV ray that will lessen the picture's contrast. if u use lens hood only without filter, the lens glass element is still exposed
expensive filters are B+W brand, nikon brand, etc. more affordable brands are HOYA. within HOYA itself it has mny different grade of filter. to save cost, a simple normal hoya UV filter will do. u wont see that much of a difference between cheap filter and branded ones
tink about it....if say u are walking and ya lens bump onto a branch.....instead of the front element getting hit, it filter will take the damage instead.....more worth it to pay for a new filter than a new lens rite..