The Unity Web Player plugin and ActiveX Controls both have a function, SendMessage(), that can be called from a web page in order to call functions within Unity web player content. This function is very similar to the GameObject.SendMessage function in the Unity scripting API. When called from a web page you pass an object name, a function name and a single argument, and SendMessage() will call the given function in the given game object.
Inside of the Unity web player content you need to have a script attached to the GameObject named MyObject, and that script needs to implement a function named MyFunction:
function MyFunction(param : String)
A single string, integer or float argument must be passed when using SendMessage(), the parameter is required on the calling side. If you don't need it then just pass a zero or other default value and ignore it on the Unity side. Additionally, the game object specified by the name can be given in the form of a path name. For example, /MyObject/SomeChild where SomeChild must be a child of MyObject and MyObject must be at the root level due to the '/' in front of its name.