Favlet and bookmarklet are two alternative names for the same thing derived from the fact that Internet Explorer Favourites are effectively the same thing as bookmarks in other browsers.
Before you grab a favlet to add to your browser, you might first want to test the effect of the favlet on the page where it is supplied. You do this by selecting the link that the favlet is attached to. This will execute the favlet against the page containing it and assuming that the page contains something that the favlet affects, you will see what the favlet does. If the page doesn't contain anything that the favlet affects then you will still not know whether the favlet works or not. Testing the favlet like this is the only time that you would left click on the favlet link.
There are two ways to attach a favlet to your browser. The simplest if you have the Links or Personal Toolbar showing in your browser window is to simply right click the favlet and drag it onto the toolbar. The alternative is to right click the favlet link (or use the keyboard to select the link and then access the context menu). You then select Add to Favorites (in Internet Explorer), File Bookmark for Link (in Netscape 6+), or Add Link to Bookmarks (in Opera) to transfer the favlet to your favorites/bookmarks menu.
Enough discussion of favelets, let's take a look at an actual favlet so that you can see for yourself how simple it is to add one to your browser and use it.
Most favelets will not give you as detailed a description of what the favlet does and its limitations as I have just given you for this one so you may have to find out by trial and error just exactly what the favlet can and can't do before you decide whether the favelet is as useful as it might at first appear to be.
The actual limit for the length of a favlet as far as I am able to determine is 606 characters. Favelets longer than this may work in some browsers but will be truncated (and therefore not work) in others. From what I have read, this limitation does not appear to apply on the Apple Mac and so many favelets developed on the Mac will not work on Windows because they are too big.
This article written by Stephen Chapman, Felgall Pty Ltd.