Remember to always use the hyphen character (-) in your aliases. That way, you'll get more value from your URL keywords. Google treats hyphen (-) like a space, while it treats underscore as a non-breaking space.
For example, "joomla-tutorials" in a URL is interpreted as two words: "joomla tutorials".
However "joomla_tutorials" is treated like one word, i.e. "joomlatutorials".
Certainly you want Google and the other search engines to recognize your keywords as cleanly as possible. Thus, always-use-the-hyphen-to-space-words-in-urls :)
The slash character / is also treated as a space, by the way. I wouldn't use that in a URL, but you are safe using it to separate two keywords in your texts. Example: "Components/modules" is treated as "components modules".