The Category Block will show a list of categories in two columns, with a category icon besides each category.
If your site has a large amount of categories and you dont want a huge list filling up your index of your site, you can use one of the alternate category widgets.
Two other Category Block Widgets are available.
Popular Categories (being posts with most categories) this list is sorted by count.
Popular Categories and Sorted (this will also list the top categories but it will by sorted in alphabetical order)