Yes, but only if you don't let attendees edit the invitation (don't select the Guests can modify event check box). Note that, as the host, you always can edit the invitation, the links won't appear as "live" on your own calendar. However, you'll see the live links in the event confirmation message you receive.
To create a link to a website in the event description, simply type the URL, such as http://www.google.com.
You can also create linked text by using HTML tags. For example, to create the link "Product Info", you would use HTML code in the format <a href="www.some-url.com">Product Info</a>. If the HTML code is valid, Google Calendar hides the HTML when you save the event (but shows the text "Product Info"). It then creates the link in the invitation that attendees receive.