A small (95x32) pixel image (PNG, GIF or JPG) that is inside a Freesite's mapfile and is used to link to that Freesite.
Why use a remote graphic instead of a normal text hyperlink? Well that works too of course, but the action of fetching the graphic from the site sort of serves as a pre-caching mechanism. It's a bit silly but it does help sites propogate.
Why 95 x 32 pixels? Nobody seems to know, they just are : ) Of course they could be different sizes but this is the standard, and your activelink probably won't get used on portals like The Freedom Engine unless you conform to it.
Also the convention is that it should be called "activelink.png", or .jpg or .gif or whatever. Some Freesite authors feel their artistic creativity is limited by 95 x 32 pixels and so make bigger alternative activelinks too for people who want to use them, which is fine, but the one called activelink.xxx should be 95 x 32.
This is an example of an animated Activelink:
