Sometimes you might want to quote a small part of an email in your reply. This can be achieved with a handy Gmail Lab. (It was originally launched as a standard Gmail feature, but later moved into Labs).
To activate the feature, go to Labs in Gmail (via Settings) and find Quote selected text in the list. Make sure Enabled is selected, and remember to save.
Now, when you want to quote an excerpt from an incoming email in your reply, simply highlight the text you want included:
When you click on Reply, the content entry field will already have the highlighted text included as a quote:
You can edit the quoted text before sending your reply, if you need to. This means that you can optionally quote multiple passages of text from the incoming email, by highlighting a larger block of content (containing all of the material you want to quote) and then editing it down before sending your reply.