How can I easily tag transactions with similar descriptions?

You can use Rules to automatically apply a tag to any transaction that matches a set of filters (like keywords in the description).

Let's say you want all purchases you make at a store named "Safeway" to be tagged as grocery. You can accomplish this by creating a tagging rule with the tag grocery and keyword safeway. Any future transactions which contain the word safeway in their description will be automatically tagged as grocery.

To create a rule, click Add > Rule.