A category allows you to group similar blog posts together. For example, if you are writing a travel blog, you could use the word 'America' as a category. Any other posts you write related to America can then be grouped under the same category
A tag makes a blog post easier to find, and provides more information on the content of your posts. Tag words that your site visitors might use when searching. For example, someone searching for information on visiting America might search for 'America', 'USA', 'Travelling America'
A blog post can have any number of tags but only one category