Enter the text you want to be tweeted in the "Text To Tweet" field. In "Link Text" you can enter a text that should appear as the name of the link. If you then press "Create Links", the "Twitter This URL" field should contain the "Tweet This" link. For your convenience the "HTML Link Code" field contains a full HTML link element you can Copy+Paste into your web site (make sure you copy all of it, I recommend right clicking into the field and choosing "select all"). "Link Preview" shows you how the link should look on your web site.
Nicole Simon has brought to my attention that creating "Twitter This" links is not as trivial as it might seem. I hope to remedy the situation by providing a simple tool for creating such links.
What is a "Twitter This" link? Simply, a link that brings the user to the Twitter page with a prepared status text field. All the user has to do to tweet your prepared text is to click the "Update" button. For example, if you click this link, you should be transferred to Twitter with the prepared status message
A handy form for generating "Twitter This" links, created by @Fractality : http://bit.ly/twitterthislinks
(Following this link does NOT tweet the text, you still have to press "Update". Why don't you go ahead and just tweet it - thanks! :-)). It even works if you are not currently logged in to Twitter. After you login, you should also see the prepared status update form. Alas, it does not (yet) work if the user has no Twitter account yet and decides to sign up on the spot. After the signup process, the text is lost (but if the user clicks on your link again, it will then work).
In my opinion, this is the preferred way to encourage users to tweet about you. Many Twitter applications ask for the user's login credentials instead and then proceed to post in the user's name. Admittedly, sometimes that might seem more effective because users might not even realize it is happening at first - so they will tweet about you even if they never intended to do so. But I consider it a slightly dirty trick, which might alienate users in the long run. Also, personally I have never given my login credentials to a Twitter app (OK, maybe once or twice, but I regret that now), and many users might feel the same.
How does one create such links? It is rather simple: add a parameter named "status" to the Twitter URL, like this: http://twitter.com?status=your_message, where your_message is your message. The minor issue is that your_message has to be URL encoded. That means certain characters in the message have to be escaped, for example "<" becomes "%3C". Most programming languages have utility methods to do this. For example you could simple paste the following snippet into your browser's URL field and press return:
(except you have to be careful about ' and ", so that approach is not 100% failsafe).
To make it easier for you, I have created a simple form for creating such URLs, which now resides at the top of this post.