gmailgmail-labels
In Gmail you have to type in a label, click it to accept it, then click labels again to type in another one…etc. etc. for all the labels you want to apply.
Is there a way to apply multiple in less steps — for example the way you might apply multiple tags using a comma?
I've never run across a way to create labels like you asked, but you can use gmail's search to do the same thing. For example, you can search for:
subject:Monkeys
Which will return all emails with Monkeys in the subject. You can also set up bookmarks for common searches.
Best Answer
In addition to the good options Ian offered of dragging labels to the emails and using the list with the mouse as a multi-selector, there is also a way to speek this up with the keyboard.
While typing labels you can accept the current one by pressing enter instead of switching to the mouse to click. You do NOT have to even type the full label, as you type the nearest match will be accepted on enter. If you have keyboard shortcuts turned on in your gmail you can label any open message or selection of multiple messages by pressing the l key. If your labels are unique enough that they don't usually share a first letter, this get's all labeling operations down to three key strokes. l > X > Return where X is the first letter of your label. L enters label mode, start typing until you have a label match, then enter to accept. Repeat to add another label by hitting L again.