Moneydance has a feature called "tags" which are similar to Quicken's "classes". You can apply multiple tags to each transaction.
In Moneydance 2015, you can simply type the name of the tag in the tags field when creating a new transaction. If you have previously used a tag, Moneydance will start to autofill the tag name after you enter in a few letters.
In versions older than Moneydance 2015, in order to edit your list of available tags, click the Tools->Edit Transaction Tags menu. You can then create tags for transactions using the popup field in the transaction edit form. There is no way to directly create tags from the transaction in versions prior to 2015.