Here’s an example link you can use to see the format:
https://www.google.com/calendar/render?action=TEMPLATE&text=Your+Event+Name&dates=20140127T224000Z/20140320T221500Z&details=For+details,+link+here:+http://www.example.com&location=Waldorf+Astoria,+301+Park+Ave+,+New+York,+NY+10022&sf=true&output=xml
Note the key query parameters:
text
dates
details
location
Here’s another example (taken from http://wordpress.org/support/topic/direct-link-to-add-specific-google-calendar-event):
<a href="http://www.google.com/calendar/render?
action=TEMPLATE
&text=[event-title]
&dates=[start-custom format="Ymd\\THi00\\Z"]/[end-custom format="Ymd\\THi00\\Z"]
&details=Link to add to Google calendar
&location=[location]
&trp=false
&sprop=
&sprop=name:"
target="_blank" rel="nofollow">Add to my calendar</a>
Here’s a form which will help you construct such a link if you want (mentioned in earlier answers):
https://support.google.com/calendar/answer/3033039
Edit: This link no longer gives you a form you can use