On the Transactions screen click on the Tag name for any transaction. A pop will appear with a list of top level category tags.
Clicking on the arrow to the right of a particular category will open the a further list of sub-tags relevant to that top level category. In some instances clicking though again will produce a third level of sub-tags. Eg, clicking through from the 'Bills' category will show a list of further tags such as Council Tax, Energy/Gas/Elec, Mortgage or Rent, TV Licence, etc.
For even more detail clicking through from Mortgage or Rent will show two further specific sub-tags for Mortgage and Rent.
If you don't know where a particular tag is located, or if you know but want to find it quickly, you can search for it typing in the search field at the top of the pop.
Apply to Similar
So you never have to tag new transactions again!
In both cases above remember to tick the 'Tag similar transactions?' checkbox before selecting the tag if you want transactions with similar/same descriptions to be tagged the same. This will not only apply the same tag to existing transactions, it will apply the tag to transactions with similar/the same description that come into your Money Dashboard in the future.