If you're looking for a certain Nyan for example: One that appears on Nyan.Cat, look in the subcategory "Appears on Nyan.Cat".
Sort by Style is useful for those who only want nyans such as non-cats, or a different body instead of a poptart. Sort by Music is good for finding a nyan that has anything but the original song or in case you only want nyans with the original song. Sort by Theme is for nyans that are based on a certain type of thing, such as Countries, Memes, Video Games etc.
Have a nyan of a time on Nyan Cat Wiki!