Get your typin' fingers warmed up, it's modifying time!
The + sign, or the word and will find all your search terms.
For example, if you type:
Cat and Dog
Cat + Dog
You'll find photos that are tagged with both the words "cat" and "dog"
The - sign, or the word not can be used to limit your search.
Cat not Dog
Cat - Dog
will find photos that are tagged with the word "cat" but not "dog."
You can use the | or the word or to be a little more flexible.
Cat or Dog
Cat | Dog
Will find photos that are tagged with the word "cat," as well as photos tagged with the word "dog" but not photos tagged with both. (Does your head hurt yet?)
Finally, if you wanna go totally bonkers, you can combine these modifiers with brackets too,
(Cat or Dog) not Pig
Will return photos tagged with the words "cat" or "dog" (not both) but not photos tagged with the word "pig." Mamma mia.